From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Date: Wed, 25 May 2022 08:44:18 -0400 Message-ID: References: <20220524234531.1949-1-peterx@redhat.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20210112.gappssmtp.com; s=20210112; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=w0F6iR9EKs/zIprBUHWYk0xdfCPICb6b6FOC3ooI/Qg=; b=LMEKNyEIRYPV5QWJlwplZCx+Yze+80z3MALcLFPz5jKdddgbR28N/X6bo9Jvgd00MP WWkcUcb1+z91voFM8WjtduJpm5pvevvusNqH0PzjaxsPqwHpEmvLG6H17fO8d+8uqQ/c xW/rbo+trH8MjL1X/foXOCmesnHHx99Tmug43Nb91EXZFTR7KAdlglnJRYtL1NUTbXz5 2CGVavB8u5eJ5bdNx8JVu0TNi6KB2NiCnWSVtKfIEdRU3i4wp/Kah2nDHcLAJXmplYfJ xzofRGg4rofGSre9mJv/qm1XS4Pms5gIR+r4L8hjS8Owdd3qkT56i/sicOD/din7w+oO VJFw== Content-Disposition: inline In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, 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 On Tue, May 24, 2022 at 07:45:31PM -0400, 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 Acked-by: Johannes Weiner