gem5/src/mem/cache/prefetch/queued.cc

/*
 * Copyright (c) 2014-2015 ARM Limited
 * All rights reserved
 *
 * The license below extends only to copyright in the software and shall
 * not be construed as granting a license to any other intellectual
 * property including but not limited to intellectual property relating
 * to a hardware implementation of the functionality of the software
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#include "mem/cache/prefetch/queued.hh"

#include <cassert>

#include "arch/generic/tlb.hh"
#include "base/logging.hh"
#include "base/trace.hh"
#include "debug/HWPrefetch.hh"
#include "mem/cache/base.hh"
#include "mem/request.hh"
#include "params/QueuedPrefetcher.hh"

namespace Prefetcher {

void
Queued::DeferredPacket::createPkt(Addr paddr, unsigned blk_size,
                                            MasterID mid, bool tag_prefetch,
                                            Tick t) {
    /* Create a prefetch memory request */
    RequestPtr req = std::make_shared<Request>(paddr, blk_size, 0, mid);

    if (pfInfo.isSecure()) {
        req->setFlags(Request::SECURE);
    }
    req->taskId(ContextSwitchTaskId::Prefetcher);
    pkt = new Packet(req, MemCmd::HardPFReq);
    pkt->allocate();
    if (tag_prefetch && pfInfo.hasPC()) {
        // Tag prefetch packet with  accessing pc
        pkt->req->setPC(pfInfo.getPC());
    }
    tick = t;
}

void
Queued::DeferredPacket::startTranslation(BaseTLB *tlb)
{
    assert(translationRequest != nullptr);
    if (!ongoingTranslation) {
        ongoingTranslation = true;
        // Prefetchers only operate in Timing mode
        tlb->translateTiming(translationRequest, tc, this, BaseTLB::Read);
    }
}

void
Queued::DeferredPacket::finish(const Fault &fault,
    const RequestPtr &req, ThreadContext *tc, BaseTLB::Mode mode)
{
    assert(ongoingTranslation);
    ongoingTranslation = false;
    bool failed = (fault != NoFault);
    owner->translationComplete(this, failed);
}

Queued::Queued(const QueuedPrefetcherParams *p)
    : Base(p), queueSize(p->queue_size),
      missingTranslationQueueSize(
        p->max_prefetch_requests_with_pending_translation),
      latency(p->latency), queueSquash(p->queue_squash),
      queueFilter(p->queue_filter), cacheSnoop(p->cache_snoop),
      tagPrefetch(p->tag_prefetch),
      throttleControlPct(p->throttle_control_percentage)
{
}

Queued::~Queued()
{
    // Delete the queued prefetch packets
    for (DeferredPacket &p : pfq) {
        delete p.pkt;
    }
}

size_t
Queued::getMaxPermittedPrefetches(size_t total) const
{
    /**
     * Throttle generated prefetches based in the accuracy of the prefetcher.
     * Accuracy is computed based in the ratio of useful prefetches with
     * respect to the number of issued prefetches.
     *
     * The throttleControlPct controls how many of the candidate addresses
     * generated by the prefetcher will be finally turned into prefetch
     * requests
     * - If set to 100, all candidates can be discarded (one request
     *   will always be allowed to be generated)
     * - Setting it to 0 will disable the throttle control, so requests are
     *   created for all candidates
     * - If set to 60, 40% of candidates will generate a request, and the
     *   remaining 60% will be generated depending on the current accuracy
     */

    size_t max_pfs = total;
    if (total > 0 && issuedPrefetches > 0) {
        size_t throttle_pfs = (total * throttleControlPct) / 100;
        size_t min_pfs = (total - throttle_pfs) == 0 ?
            1 : (total - throttle_pfs);
        max_pfs = min_pfs + (total - min_pfs) *
            usefulPrefetches / issuedPrefetches;
    }
    return max_pfs;
}

void
Queued::notify(const PacketPtr &pkt, const PrefetchInfo &pfi)
{
    Addr blk_addr = blockAddress(pfi.getAddr());
    bool is_secure = pfi.isSecure();

    // Squash queued prefetches if demand miss to same line
    if (queueSquash) {
        auto itr = pfq.begin();
        while (itr != pfq.end()) {
            if (itr->pfInfo.getAddr() == blk_addr &&
                itr->pfInfo.isSecure() == is_secure) {
                delete itr->pkt;
                itr = pfq.erase(itr);
            } else {
                ++itr;
            }
        }
    }

    // Calculate prefetches given this access
    std::vector<AddrPriority> addresses;
    calculatePrefetch(pfi, addresses);

    // Get the maximu number of prefetches that we are allowed to generate
    size_t max_pfs = getMaxPermittedPrefetches(addresses.size());

    // Queue up generated prefetches
    size_t num_pfs = 0;
    for (AddrPriority& addr_prio : addresses) {

        // Block align prefetch address
        addr_prio.first = blockAddress(addr_prio.first);

        if (!samePage(addr_prio.first, pfi.getAddr())) {
            pfSpanPage += 1;
        }

        bool can_cross_page = (tlb != nullptr);
        if (can_cross_page || samePage(addr_prio.first, pfi.getAddr())) {
            PrefetchInfo new_pfi(pfi,addr_prio.first);
            pfIdentified++;
            DPRINTF(HWPrefetch, "Found a pf candidate addr: %#x, "
                    "inserting into prefetch queue.\n", new_pfi.getAddr());
            // Create and insert the request
            insert(pkt, new_pfi, addr_prio.second);
            num_pfs += 1;
            if (num_pfs == max_pfs) {
                break;
            }
        } else {
            DPRINTF(HWPrefetch, "Ignoring page crossing prefetch.\n");
        }
    }
}

PacketPtr
Queued::getPacket()
{
    DPRINTF(HWPrefetch, "Requesting a prefetch to issue.\n");

    if (pfq.empty()) {
        // If the queue is empty, attempt first to fill it with requests
        // from the queue of missing translations
        processMissingTranslations(queueSize);
    }

    if (pfq.empty()) {
        DPRINTF(HWPrefetch, "No hardware prefetches available.\n");
        return nullptr;
    }

    PacketPtr pkt = pfq.front().pkt;
    pfq.pop_front();

    pfIssued++;
    issuedPrefetches += 1;
    assert(pkt != nullptr);
    DPRINTF(HWPrefetch, "Generating prefetch for %#x.\n", pkt->getAddr());

    processMissingTranslations(queueSize - pfq.size());
    return pkt;
}

void
Queued::regStats()
{
    Base::regStats();

    pfIdentified
        .name(name() + ".pfIdentified")
        .desc("number of prefetch candidates identified");

    pfBufferHit
        .name(name() + ".pfBufferHit")
        .desc("number of redundant prefetches already in prefetch queue");

    pfInCache
        .name(name() + ".pfInCache")
        .desc("number of redundant prefetches already in cache/mshr dropped");

    pfRemovedFull
        .name(name() + ".pfRemovedFull")
        .desc("number of prefetches dropped due to prefetch queue size");

    pfSpanPage
        .name(name() + ".pfSpanPage")
        .desc("number of prefetches that crossed the page");
}


void
Queued::processMissingTranslations(unsigned max)
{
    unsigned count = 0;
    iterator it = pfqMissingTranslation.begin();
    while (it != pfqMissingTranslation.end() && count < max) {
        DeferredPacket &dp = *it;
        // Increase the iterator first because dp.startTranslation can end up
        // calling finishTranslation, which will erase "it"
        it++;
        dp.startTranslation(tlb);
        count += 1;
    }
}

void
Queued::translationComplete(DeferredPacket *dp, bool failed)
{
    auto it = pfqMissingTranslation.begin();
    while (it != pfqMissingTranslation.end()) {
        if (&(*it) == dp) {
            break;
        }
        it++;
    }
    assert(it != pfqMissingTranslation.end());
    if (!failed) {
        DPRINTF(HWPrefetch, "%s Translation of vaddr %#x succeeded: "
                "paddr %#x \n", tlb->name(),
                it->translationRequest->getVaddr(),
                it->translationRequest->getPaddr());
        Addr target_paddr = it->translationRequest->getPaddr();
        // check if this prefetch is already redundant
        if (cacheSnoop && (inCache(target_paddr, it->pfInfo.isSecure()) ||
                    inMissQueue(target_paddr, it->pfInfo.isSecure()))) {
            pfInCache++;
            DPRINTF(HWPrefetch, "Dropping redundant in "
                    "cache/MSHR prefetch addr:%#x\n", target_paddr);
        } else {
            Tick pf_time = curTick() + clockPeriod() * latency;
            it->createPkt(it->translationRequest->getPaddr(), blkSize,
                    masterId, tagPrefetch, pf_time);
            addToQueue(pfq, *it);
        }
    } else {
        DPRINTF(HWPrefetch, "%s Translation of vaddr %#x failed, dropping "
                "prefetch request %#x \n", tlb->name(),
                it->translationRequest->getVaddr());
    }
    pfqMissingTranslation.erase(it);
}

bool
Queued::alreadyInQueue(std::list<DeferredPacket> &queue,
                                 const PrefetchInfo &pfi, int32_t priority)
{
    bool found = false;
    iterator it;
    for (it = queue.begin(); it != queue.end() && !found; it++) {
        found = it->pfInfo.sameAddr(pfi);
    }

    /* If the address is already in the queue, update priority and leave */
    if (it != queue.end()) {
        pfBufferHit++;
        if (it->priority < priority) {
            /* Update priority value and position in the queue */
            it->priority = priority;
            iterator prev = it;
            while (prev != queue.begin()) {
                prev--;
                /* If the packet has higher priority, swap */
                if (*it > *prev) {
                    std::swap(*it, *prev);
                    it = prev;
                }
            }
            DPRINTF(HWPrefetch, "Prefetch addr already in "
                "prefetch queue, priority updated\n");
        } else {
            DPRINTF(HWPrefetch, "Prefetch addr already in "
                "prefetch queue\n");
        }
    }
    return found;
}

RequestPtr
Queued::createPrefetchRequest(Addr addr, PrefetchInfo const &pfi,
                                        PacketPtr pkt)
{
    RequestPtr translation_req = std::make_shared<Request>(
            addr, blkSize, pkt->req->getFlags(), masterId, pfi.getPC(),
            pkt->req->contextId());
    translation_req->setFlags(Request::PREFETCH);
    return translation_req;
}

void
Queued::insert(const PacketPtr &pkt, PrefetchInfo &new_pfi,
                         int32_t priority)
{
    if (queueFilter) {
        if (alreadyInQueue(pfq, new_pfi, priority)) {
            return;
        }
        if (alreadyInQueue(pfqMissingTranslation, new_pfi, priority)) {
            return;
        }
    }

    /*
     * Physical address computation
     * if the prefetch is within the same page
     *   using VA: add the computed stride to the original PA
     *   using PA: no actions needed
     * if we are page crossing
     *   using VA: Create a translaion request and enqueue the corresponding
     *       deferred packet to the queue of pending translations
     *   using PA: use the provided VA to obtain the target VA, then attempt to
     *     translate the resulting address
     */

    Addr orig_addr = useVirtualAddresses ?
        pkt->req->getVaddr() : pkt->req->getPaddr();
    bool positive_stride = new_pfi.getAddr() >= orig_addr;
    Addr stride = positive_stride ?
        (new_pfi.getAddr() - orig_addr) : (orig_addr - new_pfi.getAddr());

    Addr target_paddr;
    bool has_target_pa = false;
    RequestPtr translation_req = nullptr;
    if (samePage(orig_addr, new_pfi.getAddr())) {
        if (useVirtualAddresses) {
            // if we trained with virtual addresses,
            // compute the target PA using the original PA and adding the
            // prefetch stride (difference between target VA and original VA)
            target_paddr = positive_stride ? (pkt->req->getPaddr() + stride) :
                (pkt->req->getPaddr() - stride);
        } else {
            target_paddr = new_pfi.getAddr();
        }
        has_target_pa = true;
    } else {
        // Page crossing reference

        // ContextID is needed for translation
        if (!pkt->req->hasContextId()) {
            return;
        }
        if (useVirtualAddresses) {
            has_target_pa = false;
            translation_req = createPrefetchRequest(new_pfi.getAddr(), new_pfi,
                                                    pkt);
        } else if (pkt->req->hasVaddr()) {
            has_target_pa = false;
            // Compute the target VA using req->getVaddr + stride
            Addr target_vaddr = positive_stride ?
                (pkt->req->getVaddr() + stride) :
                (pkt->req->getVaddr() - stride);
            translation_req = createPrefetchRequest(target_vaddr, new_pfi,
                                                    pkt);
        } else {
            // Using PA for training but the request does not have a VA,
            // unable to process this page crossing prefetch.
            return;
        }
    }
    if (has_target_pa && cacheSnoop &&
            (inCache(target_paddr, new_pfi.isSecure()) ||
            inMissQueue(target_paddr, new_pfi.isSecure()))) {
        pfInCache++;
        DPRINTF(HWPrefetch, "Dropping redundant in "
                "cache/MSHR prefetch addr:%#x\n", target_paddr);
        return;
    }

    /* Create the packet and find the spot to insert it */
    DeferredPacket dpp(this, new_pfi, 0, priority);
    if (has_target_pa) {
        Tick pf_time = curTick() + clockPeriod() * latency;
        dpp.createPkt(target_paddr, blkSize, masterId, tagPrefetch, pf_time);
        DPRINTF(HWPrefetch, "Prefetch queued. "
                "addr:%#x priority: %3d tick:%lld.\n",
                new_pfi.getAddr(), priority, pf_time);
        addToQueue(pfq, dpp);
    } else {
        // Add the translation request and try to resolve it later
        dpp.setTranslationRequest(translation_req);
        dpp.tc = cache->system->getThreadContext(translation_req->contextId());
        DPRINTF(HWPrefetch, "Prefetch queued with no translation. "
                "addr:%#x priority: %3d\n", new_pfi.getAddr(), priority);
        addToQueue(pfqMissingTranslation, dpp);
    }
}

void
Queued::addToQueue(std::list<DeferredPacket> &queue,
                             DeferredPacket &dpp)
{
    /* Verify prefetch buffer space for request */
    if (queue.size() == queueSize) {
        pfRemovedFull++;
        /* Lowest priority packet */
        iterator it = queue.end();
        panic_if (it == queue.begin(),
            "Prefetch queue is both full and empty!");
        --it;
        /* Look for oldest in that level of priority */
        panic_if (it == queue.begin(),
            "Prefetch queue is full with 1 element!");
        iterator prev = it;
        bool cont = true;
        /* While not at the head of the queue */
        while (cont && prev != queue.begin()) {
            prev--;
            /* While at the same level of priority */
            cont = prev->priority == it->priority;
            if (cont)
                /* update pointer */
                it = prev;
        }
        DPRINTF(HWPrefetch, "Prefetch queue full, removing lowest priority "
                            "oldest packet, addr: %#x\n",it->pfInfo.getAddr());
        delete it->pkt;
        queue.erase(it);
    }

    if (queue.size() == 0) {
        queue.emplace_back(dpp);
    } else {
        iterator it = queue.end();
        do {
            --it;
        } while (it != queue.begin() && dpp > *it);
        /* If we reach the head, we have to see if the new element is new head
         * or not */
        if (it == queue.begin() && dpp <= *it)
            it++;
        queue.insert(it, dpp);
    }
}

} // namespace Prefetcher