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Wed, 17 Jun 2020 09:10:51 -0700 (PDT) Date: Wed, 17 Jun 2020 12:10:49 -0400 From: Peter Xu To: Will Deacon Cc: Michael Ellerman , Linus Torvalds , Linux Kernel Mailing List , Gerald Schaefer , Andrew Morton , Andrea Arcangeli , openrisc@lists.librecores.org, linux-arch , Alexander Gordeev , linux-s390 , Catalin Marinas , Linux ARM Subject: Re: [PATCH 00/25] mm: Page fault accounting cleanups Message-ID: <20200617161049.GE76766@xz-x1> References: <20200615221607.7764-1-peterx@redhat.com> <87imfqecjx.fsf@mpe.ellerman.id.au> <20200617080405.GA3208@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200617080405.GA3208@willie-the-truck> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 17, 2020 at 09:04:06AM +0100, Will Deacon wrote: > On Wed, Jun 17, 2020 at 10:55:14AM +1000, Michael Ellerman wrote: > > Linus Torvalds writes: > > > On Mon, Jun 15, 2020 at 3:16 PM Peter Xu wrote: > > >> This series tries to address all of them by introducing mm_fault_accounting() > > >> first, so that we move all the page fault accounting into the common code base, > > >> then call it properly from arch pf handlers just like handle_mm_fault(). > > > > > > Hmm. > > > > > > So having looked at this a bit more, I'd actually like to go even > > > further, and just get rid of the per-architecture code _entirely_. > > > > > > > > > One detail worth noting: I do wonder if we should put the > > > > > > perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr); > > > > > > just in the arch code at the top of the fault handling, and consider > > > it entirely unrelated to the major/minor fault handling. The > > > major/minor faults fundamnetally are about successes. But the plain > > > PERF_COUNT_SW_PAGE_FAULTS could be about things that fail, including > > > things that never even get to this point at all. > > > > Yeah I think we should keep it in the arch code at roughly the top. > > I agree. It's a nice idea to consolidate the code, but I don't see that > it's really possible for PERF_COUNT_SW_PAGE_FAULTS without significantly > changing the semantics (and a potentially less useful way. Of course, > moving more of do_page_fault() out of the arch code would be great, but > that's a much bigger effort. > > > If it's moved to the end you could have a process spinning taking bad > > page faults (and fixing them up), and see no sign of it from the perf > > page fault counters. > > The current arm64 behaviour is that we record PERF_COUNT_SW_PAGE_FAULTS > if _all_ of the following are true: > > 1. The fault isn't handled by kprobes > 2. The pagefault handler is enabled > 3. We have an mm (current->mm) > 4. The fault isn't an unexpected kernel fault on a user address (we oops > and kill the task in this case) > > Which loosely corresponds to "we took a fault on a user address that it > looks like we can handle". > > That said, I'm happy to tweak this if it brings us into line with other > architectures. I see. I'll keep the semantics for PERF_COUNT_SW_PAGE_FAULTS in the next version. Thanks for all the comments! -- Peter Xu