From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vineet Gupta Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Date: Wed, 25 May 2022 20:40:51 -0700 Message-ID: <8f6add25-2e8f-4533-fa42-e43db0e32f2d@kernel.org> References: <20220524234531.1949-1-peterx@redhat.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1653536455; bh=/FC3fRDbTNCC3O7WmAhxUVAkmggWpUFeLv7OpR1lxLg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=hLLrqk/dSj3ABZgHj+oG01SXfcPdfcPLbCPW5c/vtlKKjGhr826rFN6lLQogyE6R+ V4+rLm9jBH6we4JcRrEMO+7ArvhQ4vUukeFX5qG0umNfJEjOVl7t+dToTe7orX2kZK bSWoDGynO3/vHi04K7UfgRIeRSjHN5GtmmMIX2USGvftGI9pdfeJrjgnAmxrltc5n0 8y833O4MCr4JKKzVPSDGbAvYYjpDQY/dQCqYoM1yOIlCVTJtDFNiHcFx0Vs05+lAF0 ALTHrfJeOAfPcImvCexeRL9L6Mpy6p/bjPRzg8B1971s8Zq8YHz1AyoKFvxIGL0AWO PHAjKqVSOf5bw== Content-Language: en-US In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Peter Xu , linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: Richard Henderson , David Hildenbrand , Matt Turner , Albert Ou , Michal Simek , Russell King , Ivan Kokshaysky , linux-riscv@lists.infradead.org, Alexander Gordeev , Dave Hansen , Jonas Bonn , Will Deacon , "James E . J . Bottomley" , "H . Peter Anvin" , Andrea Arcangeli , openrisc@lists.librecores.org, linux-s390@vger.kernel.org, Ingo Molnar , linux-m68k@lists.linux-m68k.org, Palmer Dabbelt , Heiko On 5/24/22 16:45, Peter Xu wrote: > I observed that for each of the shared file-backed page faults, we're very > likely to retry one more time for the 1st write fault upon no page. It's > because we'll need to release the mmap lock for dirty rate limit purpose > with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()). > > Then after that throttling we return VM_FAULT_RETRY. > > We did that probably because VM_FAULT_RETRY is the only way we can return > to the fault handler at that time telling it we've released the mmap lock. > > However that's not ideal because it's very likely the fault does not need > to be retried at all since the pgtable was well installed before the > throttling, so the next continuous fault (including taking mmap read lock, > walk the pgtable, etc.) could be in most cases unnecessary. > > It's not only slowing down page faults for shared file-backed, but also add > more mmap lock contention which is in most cases not needed at all. > > To observe this, one could try to write to some shmem page and look at > "pgfault" value in /proc/vmstat, then we should expect 2 counts for each > shmem write simply because we retried, and vm event "pgfault" will capture > that. > > To make it more efficient, add a new VM_FAULT_COMPLETED return code just to > show that we've completed the whole fault and released the lock. It's also > a hint that we should very possibly not need another fault immediately on > this page because we've just completed it. > > This patch provides a ~12% perf boost on my aarch64 test VM with a simple > program sequentially dirtying 400MB shmem file being mmap()ed and these are > the time it needs: > > Before: 650.980 ms (+-1.94%) > After: 569.396 ms (+-1.38%) > > I believe it could help more than that. > > We need some special care on GUP and the s390 pgfault handler (for gmap > code before returning from pgfault), the rest changes in the page fault > handlers should be relatively straightforward. > > Another thing to mention is that mm_account_fault() does take this new > fault as a generic fault to be accounted, unlike VM_FAULT_RETRY. > > I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do > not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping > them as-is. > > Signed-off-by: Peter Xu > --- > > v3: > - Rebase to akpm/mm-unstable > - Copy arch maintainers > --- > arch/arc/mm/fault.c | 4 ++++ Acked-by: Vineet Gupta Thx, -Vineet