From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Xu Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Date: Fri, 27 May 2022 10:53:36 -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=redhat.com; s=mimecast20190719; t=1653663225; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=uLEQbfXieXddbws3ZtMPUo3kSGjUH1tIUFKal4fFd0k=; b=ZUWXkMmaLR/WV6SaN23An4IjuzAIza1QX1t8JzsMALCVYB77n5xLXUBVTeDmXG4i9Rz8In 93SdTNCn3lvYQKzM1zI+gr5m3+U3u6Q3xuOoj9aE3GpQX8tvfmVao021lGneA/iU37e0Wi G7M7d7KrJBIJ1UlhRAr3ZuYKmDOg12A= Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Ingo Molnar 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 Fri, May 27, 2022 at 12:46:31PM +0200, Ingo Molnar wrote: > > * Peter Xu wrote: > > > 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%) > > Nice! > > > arch/x86/mm/fault.c | 4 ++++ > > Reviewed-by: Ingo Molnar > > Minor comment typo: > > > + /* > > + * We should do the same as VM_FAULT_RETRY, but let's not > > + * return -EBUSY since that's not reflecting the reality on > > + * what has happened - we've just fully completed a page > > + * fault, with the mmap lock released. Use -EAGAIN to show > > + * that we want to take the mmap lock _again_. > > + */ > > s/reflecting the reality on what has happened > /reflecting the reality of what has happened Will fix. > > > ret = handle_mm_fault(vma, address, fault_flags, NULL); > > + > > + if (ret & VM_FAULT_COMPLETED) { > > + /* > > + * NOTE: it's a pity that we need to retake the lock here > > + * to pair with the unlock() in the callers. Ideally we > > + * could tell the callers so they do not need to unlock. > > + */ > > + mmap_read_lock(mm); > > + *unlocked = true; > > + return 0; > > Indeed that's a pity - I guess more performance could be gained here, > especially in highly parallel threaded workloads? Yes I think so. The patch avoids the page fault retry, including the mmap lock/unlock side. Now if we retake the lock for fixup_user_fault() we still safe time for pgtable walks but the lock overhead will be somehow kept, just with smaller critical sections. Some fixup_user_fault() callers won't be affected as long as unlocked==NULL is passed - e.g. the futex code path (fault_in_user_writeable). After all they never needed to retake the lock before/after this patch. It's about the other callers, and they may need some more touch-ups case by case. Examples are follow_fault_pfn() in vfio and hva_to_pfn_remapped() in KVM: both of them returns -EAGAIN when *unlocked==true. We need to teach them to know "*unlocked==true" does not necessarily mean a retry attempt. I think I can look into them if this patch can be accepted as a follow up. Thanks for taking a look! -- Peter Xu