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[35.238.199.243]) by smtp.gmail.com with ESMTPSA id h13sm1857889ild.16.2021.04.11.21.03.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 11 Apr 2021 21:03:02 -0700 (PDT) Date: Mon, 12 Apr 2021 04:03:01 +0000 From: Dennis Zhou To: Wang Yugui Cc: Vlastimil Babka , linux-mm@kvack.org, linux-btrfs@vger.kernel.org Subject: Re: unexpected -ENOMEM from percpu_counter_init() Message-ID: References: <20210411000846.9CC6.409509F4@e16-tech.com> <20210411232000.BF15.409509F4@e16-tech.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210411232000.BF15.409509F4@e16-tech.com> Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On Sun, Apr 11, 2021 at 11:20:00PM +0800, Wang Yugui wrote: > Hi, Dennis Zhou > > > Hi, > > > > > On Sat, Apr 10, 2021 at 11:29:17PM +0800, Wang Yugui wrote: > > > > Hi, Dennis Zhou > > > > > > > > Thanks for your ncie answer. > > > > but still a few questions. > > > > > > > > > Percpu is not really cheap memory to allocate because it has a > > > > > amplification factor of NR_CPUS. As a result, percpu on the critical > > > > > path is really not something that is expected to be high throughput. > > > > > > > > > Ideally things like btrfs snapshots should preallocate a number of these > > > > > and not try to do atomic allocations because that in theory could fail > > > > > because even after we go to the page allocator in the future we can't > > > > > get enough pages due to needing to go into reclaim. > > > > > > > > pre-allocate in module such as mempool_t is just used in a few place in > > > > linux/fs. so most people like system wide pre-allocate, because it is > > > > more easy to use? > > > > > > > > can we add more chance to management the system wide pre-alloc > > > > just like this? > > > > > > > > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > > > > index dc1f4dc..eb3f592 100644 > > > > --- a/include/linux/sched/mm.h > > > > +++ b/include/linux/sched/mm.h > > > > @@ -226,6 +226,11 @@ static inline void memalloc_noio_restore(unsigned int flags) > > > > static inline unsigned int memalloc_nofs_save(void) > > > > { > > > > unsigned int flags = current->flags & PF_MEMALLOC_NOFS; > > > > + > > > > + // just like slab_pre_alloc_hook > > > > + fs_reclaim_acquire(current->flags & gfp_allowed_mask); > > > > + fs_reclaim_release(current->flags & gfp_allowed_mask); > > > > + > > > > current->flags |= PF_MEMALLOC_NOFS; > > > > return flags; > > > > } > > > > > > > > > > > > > The workqueue approach has been good enough so far. Technically there is > > > > > a higher priority workqueue that this work could be scheduled on, but > > > > > save for this miss on my part, the system workqueue has worked out fine. > > > > > > > > > In the future as I mentioned above. It would be good to support actually > > > > > getting pages, but it's work that needs to be tackled with a bit of > > > > > care. I might target the work for v5.14. > > > > > > > > > > > this is our application pipeline. > > > > > > file_pre_process | > > > > > > bwa.nipt xx | > > > > > > samtools.nipt sort xx | > > > > > > file_post_process > > > > > > > > > > > > file_pre_process/file_post_process is fast, so often are blocked by > > > > > > pipe input/output. > > > > > > > > > > > > 'bwa.nipt xx' is a high-cpu-load, almost all of CPU cores. > > > > > > > > > > > > 'samtools.nipt sort xx' is a high-mem-load, it keep the input in memory. > > > > > > if the memory is not enough, it will save all the buffer to temp file, > > > > > > so it is sometimes high-IO-load too(write 60G or more to file). > > > > > > > > > > > > > > > > > > xfstests(generic/476) is just high-IO-load, cpu/memory load is NOT high. > > > > > > so xfstests(generic/476) maybe easy than our application pipeline. > > > > > > > > > > > > Although there is yet not a simple reproducer for another problem > > > > > > happend here, but there is a little high chance that something is wrong > > > > > > in btrfs/mm/fs-buffer. > > > > > > > but another problem(os freezed without call trace, PANIC without OOPS?, > > > > > > > the reason is yet unkown) still happen. > > > > > > > > > > I do not have an answer for this. I would recommend looking into kdump. > > > > > > > > percpu ENOMEM problem blocked many heavy load test a little long time? > > > > I still guess this problem of system freeze is a mm/btrfs problem. > > > > OOM not work, OOPS not work too. > > > > > > > > > > I don't follow. Is this still a problem after the patch? > > > > > > After the patch for percpu ENOMEM, the problem of system freeze have a high > > frequecy (>75%) to be triggered by our user-space application. > > > > The problem of system freeze maybe not caused by the percpu ENOMEM patch. > > > > percpu ENOMEM problem maybe more easy to happen than the problem of > > system freeze. > > After highmem zone +80% / otherzone +40% of WMARK_MIN/ WMARK_LOW/ > WMARK_HIGH, we walked around or reduced the reproduce frequency of the > problem of system freeze. > > so this is a problem of linux-mm. > > the user case of our user-space application. > 1) write the files with the total size > 3 * memory size. > the memory size > 128G > 2) btrfs with SSD/SAS, SSD/SATA, or btrfs RAID6 hdd > SSD/NVMe maybe too fast, so difficult to reproduce. > 3) some CPU load, and some memory load. > To me it just sounds like writeback is slow. It's hard to debug a system without actually observing it as well. You might want to limit the memory allotted to the workload cgroup possibly memory.high. This may help kick reclaim in earlier. > btrfs and other fs seem not like mempool_t wiht pre-alloc, so difficult > job is left to the system-wide reclaim/pre-alloc of linux-mm. > > maye memalloc_nofs_save() or memalloc_nofs_restore() is a good place to > add some sync/aysnc memory reclaim/pre-alloc operations for WMARK_MIN/ > WMARK_LOW/WMARK_HIGH and percpu PCPU_EMPTY_POP_PAGES_LOW. > It's not that simple. Memory reclaim is a balancing act and these places mark where reclaim cannot trigger writeback and thus oom-killer is the only way out. I'm sorry, but beyond the above, I don't really have any additional advice besides retuning your workload to use less memory and give the system more headroom. I appreciate the bug report though and if its anything percpu related I will always be available. > Best Regards > Wang Yugui (wangyugui@e16-tech.com) > 2021/04/11 > Thanks, Dennis