From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Date: Wed, 25 May 2022 13:10:17 +0200 Message-ID: References: <20220524234531.1949-1-peterx@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=CY7qtAv1oTJczHETHbhC9CMEk0chrOG8tzOjmljXr8k=; b=CF3ZPBvrJgOtga OJ1tzv6x8iMNYo/XmcpO2r1Ff1TTv4jQGlrQxoILcrgzXfEMefepvVuFAmxfq1SW9ANl1jm0y2+dY ojPngvJW2k02MWe6Vs4ibgLdc0eta1E+Lno503vtUTlu1ZEn7JuQJOfeG1wc6pKzRhwjXxLG4+h4B VJePtqBSg/DraNdmvhMTb0JZUhoMI8o7zQdmKaujNp50MrTvP9ZSr207b2q/QYG09c79Z+UbaJ/ac WnbP87amnrsJcHP52XlPs/X1RfNMAx1rH56jf+GrQmHCkFBNQeuyLsV7cc7Wa+lU0Sl22uBQeUcyH +mLdkik26RhofNzY2QWg==; DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=TTSfncRF4bwVxkeR+S4pHa2NuE5vU7NkHqU52imXoeo=; b=AZ7NLjulpP1WzqY4jAw9CJ6mzn eNxn6mITLdAkzreesFIusYyJANkaOvbZVslHzJxDDqo/jW9prJ6mSOYajnNAsWgSTZTPCGe4Y+H+t Q7b5K2i3OxODvn+feDcYZuuvagMcqlznlm2xJ6e8wPUzB4c/WkijGr50Z8D32ZlBmwWuMwGQbue1E mrsCyjPGVmj7xf/0/rnV0zIseeyVAHCeyTCYTi0ZFmZwkk1CWC0IMPFglArv51u64lwHpRP8/QuNm TypGtB34O3uGaPoduTA8k7e4jOZswva+y2onGqiOIb4Z5GAtPEnJDGpyLSzDy3ZlKvUO8MvNVaZm2 4u9mbK6A==; Content-Disposition: inline In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-riscv" Errors-To: linux-riscv-bounces+glpr-linux-riscv=m.gmane-mx.org@lists.infradead.org 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, Palmer Dabbelt , Heiko Carstens , Chris Zankel , Alistair 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: Peter Zijlstra (Intel)