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Rozycki" , linux-mm@kvack.org, David Hildenbrand Subject: Re: [PATCH v5 2/4] mm/memory.c: Update local TLB if PTE entry exists Message-Id: <20200521122211.7450025a41865a67df6a7303@linux-foundation.org> In-Reply-To: <1590031837-9582-2-git-send-email-maobibo@loongson.cn> References: <1590031837-9582-1-git-send-email-maobibo@loongson.cn> <1590031837-9582-2-git-send-email-maobibo@loongson.cn> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, 21 May 2020 11:30:35 +0800 Bibo Mao wrote: > If two threads concurrently fault at the same address, the thread that > won the race updates the PTE and its local TLB. For now, the other > thread gives up, simply does nothing, and continues. > > It could happen that this second thread triggers another fault, whereby > it only updates its local TLB while handling the fault. Instead of > triggering another fault, let's directly update the local TLB of the > second thread. > > It is only useful to architectures where software can update TLB, it may > bring out some negative effect if update_mmu_cache is used for other > purpose also. It seldom happens where multiple threads access the same > page at the same time, so the negative effect is limited on other arches. > > With specjvm2008 workload, smp-race pgfault counts is about 3% to 4% > of the total pgfault counts by watching /proc/vmstats information > I'm sorry to keep thrashing this for so long, but I'd really prefer not to add any overhead to architectures which don't need it. However, we're getting somewhere! > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2436,10 +2436,9 @@ static inline bool cow_user_page(struct page *dst, struct page *src, > if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) { > /* > * Other thread has already handled the fault > - * and we don't need to do anything. If it's > - * not the case, the fault will be triggered > - * again on the same address. > + * and update local tlb only > */ > + update_mmu_cache(vma, addr, vmf->pte); Now, all the patch does is to add new calls to update_mmu_cache(). So can we replace all these with a call to a new update_mmu_cache_sw_tlb() (or whatever) which is a no-op on architectures which don't need the additional call? Also, I wonder about the long-term maintainability. People who regularly work on this code won't be thinking of this MIPS peculiarity and it's likely that any new calls to update_mmu_cache_sw_tlb() won't be added where they should have been. Hopefully copy-and-paste from the existing code will serve us well. Please do ensure that the update_mmu_cache_sw_tlb() implementation is carefully commented so that people can understand where they should (and shouldn't) include this call.