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Sun, 23 Jan 2022 13:28:34 -0800 (PST) Date: Sun, 23 Jan 2022 14:28:30 -0700 From: Yu Zhao To: Michal Hocko Cc: Andrew Morton , Linus Torvalds , Andi Kleen , Catalin Marinas , Dave Hansen , Hillf Danton , Jens Axboe , Jesse Barnes , Johannes Weiner , Jonathan Corbet , Matthew Wilcox , Mel Gorman , Michael Larabel , Rik van Riel , Vlastimil Babka , Will Deacon , Ying Huang , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, page-reclaim@google.com, x86@kernel.org, Konstantin Kharlamov Subject: Re: [PATCH v6 6/9] mm: multigenerational lru: aging Message-ID: References: <20220104202227.2903605-1-yuzhao@google.com> <20220104202227.2903605-7-yuzhao@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: 4njxgms18jp4t7afth3na7io8gcfab5j Authentication-Results: imf08.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=m8u25CZK; spf=pass (imf08.hostedemail.com: domain of yuzhao@google.com designates 209.85.166.44 as permitted sender) smtp.mailfrom=yuzhao@google.com; dmarc=pass (policy=reject) header.from=google.com X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: E80C2160004 X-HE-Tag: 1642973315-948599 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jan 19, 2022 at 10:42:47AM +0100, Michal Hocko wrote: > On Wed 19-01-22 00:04:10, Yu Zhao wrote: > > On Mon, Jan 10, 2022 at 11:54:42AM +0100, Michal Hocko wrote: > > > On Sun 09-01-22 21:47:57, Yu Zhao wrote: > > > > On Fri, Jan 07, 2022 at 03:44:50PM +0100, Michal Hocko wrote: > > > > > On Tue 04-01-22 13:22:25, Yu Zhao wrote: > > > > > [...] > > > > > > +static void walk_mm(struct lruvec *lruvec, struct mm_struct *mm, struct lru_gen_mm_walk *walk) > > > > > > +{ > > > > > > + static const struct mm_walk_ops mm_walk_ops = { > > > > > > + .test_walk = should_skip_vma, > > > > > > + .p4d_entry = walk_pud_range, > > > > > > + }; > > > > > > + > > > > > > + int err; > > > > > > +#ifdef CONFIG_MEMCG > > > > > > + struct mem_cgroup *memcg = lruvec_memcg(lruvec); > > > > > > +#endif > > > > > > + > > > > > > + walk->next_addr = FIRST_USER_ADDRESS; > > > > > > + > > > > > > + do { > > > > > > + unsigned long start = walk->next_addr; > > > > > > + unsigned long end = mm->highest_vm_end; > > > > > > + > > > > > > + err = -EBUSY; > > > > > > + > > > > > > + rcu_read_lock(); > > > > > > +#ifdef CONFIG_MEMCG > > > > > > + if (memcg && atomic_read(&memcg->moving_account)) > > > > > > + goto contended; > > > > > > +#endif > > > > > > + if (!mmap_read_trylock(mm)) > > > > > > + goto contended; > > > > > > > > > > Have you evaluated the behavior under mmap_sem contention? I mean what > > > > > would be an effect of some mms being excluded from the walk? This path > > > > > is called from direct reclaim and we do allocate with exclusive mmap_sem > > > > > IIRC and the trylock can fail in a presence of pending writer if I am > > > > > not mistaken so even the read lock holder (e.g. an allocation from the #PF) > > > > > can bypass the walk. > > > > > > > > You are right. Here it must be a trylock; otherwise it can deadlock. > > > > > > Yeah, this is clear. > > > > > > > I think there might be a misunderstanding: the aging doesn't > > > > exclusively rely on page table walks to gather the accessed bit. It > > > > prefers page table walks but it can also fallback to the rmap-based > > > > function, i.e., lru_gen_look_around(), which only gathers the accessed > > > > bit from at most 64 PTEs and therefore is less efficient. But it still > > > > retains about 80% of the performance gains. > > > > > > I have to say that I really have hard time to understand the runtime > > > behavior depending on that interaction. How does the reclaim behave when > > > the virtual scan is enabled, partially enabled and almost completely > > > disabled due to different constrains? I do not see any such an > > > evaluation described in changelogs and I consider this to be a rather > > > important information to judge the overall behavior. > > > > It doesn't have (partially) enabled/disabled states nor does its > > behavior change with different reclaim constraints. Having either > > would make its design too complex to implement or benchmark. > > Let me clarify. By "partially enabled" I really meant behavior depedning > on runtime conditions. Say mmap_sem cannot be locked for half of scanned > tasks and/or allocation for the mm walker fails due to lack of memory. > How does this going to affect reclaim efficiency. Understood. This is not only possible -- it's the default for our ARM hardware that doesn't support the accessed bit, i.e., CPUs that don't automatically set the accessed bit. In try_to_inc_max_seq(), we have: /* * If the hardware doesn't automatically set the accessed bit, fallback * to lru_gen_look_around(), which only clears the accessed bit in a * handful of PTEs. Spreading the work out over a period of time usually * is less efficient, but it avoids bursty page faults. */ if the accessed bit is not supported return if alloc_mm_walk() fails return walk_mm() if mmap_sem contented return scan page tables We have a microbenchmark that specifically measures this worst case scenario by entirely disabling page table scanning. Its results showed that this still retains more than 90% of the optimal performance. I'll share this microbenchmark in another email when answering Barry's questions regarding the accessed bit. Our profiling infra also indirectly confirms this: it collects data from real users running on hardware with and without the accessed bit. Users running on hardware without the accessed bit indeed suffer a small performance degradation, compared with users running on hardware with it. But they still benefit almost as much, compared with users running on the same hardware but without MGLRU. > How does a user/admin > know that the memory reclaim is in a "degraded" mode because of the > contention? As we previously discussed here: https://lore.kernel.org/linux-mm/Ydu6fXg2FmrseQOn@google.com/ there used to be a counter measuring the contention, and it was deemed unnecessary and removed in v4. But I don't have a problem if we want to revive it.