From mboxrd@z Thu Jan 1 00:00:00 1970 From: Heiko Carstens Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Date: Fri, 27 May 2022 14:23:42 +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=QsHjDsOEjaVs4/QFYjWMRqYjCuAIx/EJiepWOxCjdvY=; b=1Tl0c7F1uWqDdN Wu5zbXUwwba3HVru17goIwFDM4YjA7ZMuaVnpNAAFTeYmKdtx6bYIlPLuWWdUp4AWQI5EZDH/BHJs KrIPiJsNhRKZcUWXPyWUDcUJeJwSSBJssF+yC4mMfysW3nKwMywbW5CKnECNsXhtEvhGs5VwnxOFF y19/ARql0qMjpKnVYq/Pl8TlB78t7vvJd5X9eVJbo//RdY8SAJJAUU8ON6cK/7sqNNafOJYt7eHT8 jeGKAJ6z1wegIaqiLGNi3QHxWi5ZlFvF05OK+N4x7yWwrpIfXxWfmuCYisYdZdFroW0CcIbHISzX/ ieB0XVcfK5+xABjP1+Ng==; DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=date : from : to : cc : subject : message-id : references : mime-version : content-type : in-reply-to; s=pp1; bh=RVk/+c6nPK5D2qeDI0zLbVO1lqN1tQYD0TzNeMii76M=; b=CdGIqDdzBXd3h2dRHpL1DShFdmTRQ/K3BNoowjsJ7U4uRWdEUL24qUkp9B9CCe4qwI1x ueMHE+3+v1uC7fMJZdhALNq0R69QTPr2X7t7hmmcYdqjvUF+dOgF+KjoQRzbmFzsymwU 4TRs+Fcb59XpwhBMszKHjkfxh5jERETh0DSQF2mQOARMUdDnfiEcL/AxzL9Ab0Nz2WHK SDNr7raIw4jjkxtIKRrkZp9dwvFLuLsudS4hJ9Ynj7wBxCmCSFMaztPXonoUGxJ1hjXA 3uotVOEbohqQcj6NjHHndqMeFT50tNx88feHmMQrikGztx2louDZC2dPTlWt2pNYYxwq Kg== 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 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 ... > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c > index e173b6187ad5..9503a7cfaf03 100644 > --- a/arch/s390/mm/fault.c > +++ b/arch/s390/mm/fault.c > @@ -339,6 +339,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > unsigned long address; > unsigned int flags; > vm_fault_t fault; > + bool need_unlock = true; > bool is_write; > > tsk = current; > @@ -433,6 +434,13 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > goto out_up; > goto out; > } > + > + /* The fault is fully completed (including releasing mmap lock) */ > + if (fault & VM_FAULT_COMPLETED) { > + need_unlock = false; > + goto out_gmap; > + } > + > if (unlikely(fault & VM_FAULT_ERROR)) > goto out_up; > > @@ -452,6 +460,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > mmap_read_lock(mm); > goto retry; > } > +out_gmap: > if (IS_ENABLED(CONFIG_PGSTE) && gmap) { > address = __gmap_link(gmap, current->thread.gmap_addr, > address); > @@ -466,7 +475,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > } > fault = 0; > out_up: > - mmap_read_unlock(mm); > + if (need_unlock) > + mmap_read_unlock(mm); > out: This seems to be incorrect. __gmap_link() requires the mmap_lock to be held. Christian, Janosch, or David, could you please check?