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[49.181.60.96]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3056f8d6c17sm1335390a91.41.2025.04.02.04.32.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Apr 2025 04:32:17 -0700 (PDT) Received: from dave by dread.disaster.area with local (Exim 4.98) (envelope-from ) id 1tzwKM-00000003WFl-0iwn; Wed, 02 Apr 2025 22:32:14 +1100 Date: Wed, 2 Apr 2025 22:32:14 +1100 From: Dave Chinner To: Yafang Shao Cc: Harry Yoo , Kees Cook , joel.granados@kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Josef Bacik , linux-mm@kvack.org, Vlastimil Babka Subject: Re: [PATCH] proc: Avoid costly high-order page allocations when reading proc files Message-ID: References: <20250401073046.51121-1-laoar.shao@gmail.com> <3315D21B-0772-4312-BCFB-402F408B0EF6@kernel.org> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Apr 02, 2025 at 04:42:06PM +0800, Yafang Shao wrote: > On Wed, Apr 2, 2025 at 12:15 PM Harry Yoo wrote: > > > > On Tue, Apr 01, 2025 at 07:01:04AM -0700, Kees Cook wrote: > > > > > > > > > On April 1, 2025 12:30:46 AM PDT, Yafang Shao wrote: > > > >While investigating a kcompactd 100% CPU utilization issue in production, I > > > >observed frequent costly high-order (order-6) page allocations triggered by > > > >proc file reads from monitoring tools. This can be reproduced with a simple > > > >test case: > > > > > > > > fd = open(PROC_FILE, O_RDONLY); > > > > size = read(fd, buff, 256KB); > > > > close(fd); > > > > > > > >Although we should modify the monitoring tools to use smaller buffer sizes, > > > >we should also enhance the kernel to prevent these expensive high-order > > > >allocations. > > > > > > > >Signed-off-by: Yafang Shao > > > >Cc: Josef Bacik > > > >--- > > > > fs/proc/proc_sysctl.c | 10 +++++++++- > > > > 1 file changed, 9 insertions(+), 1 deletion(-) > > > > > > > >diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c > > > >index cc9d74a06ff0..c53ba733bda5 100644 > > > >--- a/fs/proc/proc_sysctl.c > > > >+++ b/fs/proc/proc_sysctl.c > > > >@@ -581,7 +581,15 @@ static ssize_t proc_sys_call_handler(struct kiocb *iocb, struct iov_iter *iter, > > > > error = -ENOMEM; > > > > if (count >= KMALLOC_MAX_SIZE) > > > > goto out; > > > >- kbuf = kvzalloc(count + 1, GFP_KERNEL); > > > >+ > > > >+ /* > > > >+ * Use vmalloc if the count is too large to avoid costly high-order page > > > >+ * allocations. > > > >+ */ > > > >+ if (count < (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)) > > > >+ kbuf = kvzalloc(count + 1, GFP_KERNEL); > > > > > > Why not move this check into kvmalloc family? > > > > Hmm should this check really be in kvmalloc family? > > Modifying the existing kvmalloc functions risks performance regressions. > Could we instead introduce a new variant like vkmalloc() (favoring > vmalloc over kmalloc) or kvmalloc_costless()? We should fix kvmalloc() instead of continuing to force subsystems to work around the limitations of kvmalloc(). Have a look at xlog_kvmalloc() in XFS. It implements a basic fast-fail, no retry high order kmalloc before it falls back to vmalloc by turning off direct reclaim for the kmalloc() call. Hence if the there isn't a high-order page on the free lists ready to allocate, it falls back to vmalloc() immediately. For XFS, using xlog_kvmalloc() reduced the high-order per-allocation overhead by around 80% when compared to a standard kvmalloc() call. Numbers and profiles were documented in the commit message (reproduced in whole below)... > > I don't think users would expect kvmalloc() to implictly decide on using > > vmalloc() without trying kmalloc() first, just because it's a high-order > > allocation. Right, but users expect kvmalloc() to use the most efficient allocation paths available to it. In this case, vmalloc() is faster and more reliable than direct reclaim w/ compaction. Hence vmalloc() should really be the primary fallback path when high-order pages are not immediately available to kmalloc() when called from kvmalloc()... -Dave. -- Dave Chinner david@fromorbit.com commit 8dc9384b7d75012856b02ff44c37566a55fc2abf Author: Dave Chinner Date: Tue Jan 4 17:22:18 2022 -0800 xfs: reduce kvmalloc overhead for CIL shadow buffers Oh, let me count the ways that the kvmalloc API sucks dog eggs. The problem is when we are logging lots of large objects, we hit kvmalloc really damn hard with costly order allocations, and behaviour utterly sucks: - 49.73% xlog_cil_commit - 31.62% kvmalloc_node - 29.96% __kmalloc_node - 29.38% kmalloc_large_node - 29.33% __alloc_pages - 24.33% __alloc_pages_slowpath.constprop.0 - 18.35% __alloc_pages_direct_compact - 17.39% try_to_compact_pages - compact_zone_order - 15.26% compact_zone 5.29% __pageblock_pfn_to_page 3.71% PageHuge - 1.44% isolate_migratepages_block 0.71% set_pfnblock_flags_mask 1.11% get_pfnblock_flags_mask - 0.81% get_page_from_freelist - 0.59% _raw_spin_lock_irqsave - do_raw_spin_lock __pv_queued_spin_lock_slowpath - 3.24% try_to_free_pages - 3.14% shrink_node - 2.94% shrink_slab.constprop.0 - 0.89% super_cache_count - 0.66% xfs_fs_nr_cached_objects - 0.65% xfs_reclaim_inodes_count 0.55% xfs_perag_get_tag 0.58% kfree_rcu_shrink_count - 2.09% get_page_from_freelist - 1.03% _raw_spin_lock_irqsave - do_raw_spin_lock __pv_queued_spin_lock_slowpath - 4.88% get_page_from_freelist - 3.66% _raw_spin_lock_irqsave - do_raw_spin_lock __pv_queued_spin_lock_slowpath - 1.63% __vmalloc_node - __vmalloc_node_range - 1.10% __alloc_pages_bulk - 0.93% __alloc_pages - 0.92% get_page_from_freelist - 0.89% rmqueue_bulk - 0.69% _raw_spin_lock - do_raw_spin_lock __pv_queued_spin_lock_slowpath 13.73% memcpy_erms - 2.22% kvfree On this workload, that's almost a dozen CPUs all trying to compact and reclaim memory inside kvmalloc_node at the same time. Yet it is regularly falling back to vmalloc despite all that compaction, page and shrinker reclaim that direct reclaim is doing. Copying all the metadata is taking far less CPU time than allocating the storage! Direct reclaim should be considered extremely harmful. This is a high frequency, high throughput, CPU usage and latency sensitive allocation. We've got memory there, and we're using kvmalloc to allow memory allocation to avoid doing lots of work to try to do contiguous allocations. Except it still does *lots of costly work* that is unnecessary. Worse: the only way to avoid the slowpath page allocation trying to do compaction on costly allocations is to turn off direct reclaim (i.e. remove __GFP_RECLAIM_DIRECT from the gfp flags). Unfortunately, the stupid kvmalloc API then says "oh, this isn't a GFP_KERNEL allocation context, so you only get kmalloc!". This cuts off the vmalloc fallback, and this leads to almost instant OOM problems which ends up in filesystems deadlocks, shutdowns and/or kernel crashes. I want some basic kvmalloc behaviour: - kmalloc for a contiguous range with fail fast semantics - no compaction direct reclaim if the allocation enters the slow path. - run normal vmalloc (i.e. GFP_KERNEL) if kmalloc fails The really, really stupid part about this is these kvmalloc() calls are run under memalloc_nofs task context, so all the allocations are always reduced to GFP_NOFS regardless of the fact that kvmalloc requires GFP_KERNEL to be passed in. IOWs, we're already telling kvmalloc to behave differently to the gfp flags we pass in, but it still won't allow vmalloc to be run with anything other than GFP_KERNEL. So, this patch open codes the kvmalloc() in the commit path to have the above described behaviour. The result is we more than halve the CPU time spend doing kvmalloc() in this path and transaction commits with 64kB objects in them more than doubles. i.e. we get ~5x reduction in CPU usage per costly-sized kvmalloc() invocation and the profile looks like this: - 37.60% xlog_cil_commit 16.01% memcpy_erms - 8.45% __kmalloc - 8.04% kmalloc_order_trace - 8.03% kmalloc_order - 7.93% alloc_pages - 7.90% __alloc_pages - 4.05% __alloc_pages_slowpath.constprop.0 - 2.18% get_page_from_freelist - 1.77% wake_all_kswapds .... - __wake_up_common_lock - 0.94% _raw_spin_lock_irqsave - 3.72% get_page_from_freelist - 2.43% _raw_spin_lock_irqsave - 5.72% vmalloc - 5.72% __vmalloc_node_range - 4.81% __get_vm_area_node.constprop.0 - 3.26% alloc_vmap_area - 2.52% _raw_spin_lock - 1.46% _raw_spin_lock 0.56% __alloc_pages_bulk - 4.66% kvfree - 3.25% vfree - __vfree - 3.23% __vunmap - 1.95% remove_vm_area - 1.06% free_vmap_area_noflush - 0.82% _raw_spin_lock - 0.68% _raw_spin_lock - 0.92% _raw_spin_lock - 1.40% kfree - 1.36% __free_pages - 1.35% __free_pages_ok - 1.02% _raw_spin_lock_irqsave It's worth noting that over 50% of the CPU time spent allocating these shadow buffers is now spent on spinlocks. So the shadow buffer allocation overhead is greatly reduced by getting rid of direct reclaim from kmalloc, and could probably be made even less costly if vmalloc() didn't use global spinlocks to protect it's structures. Signed-off-by: Dave Chinner Reviewed-by: Allison Henderson Reviewed-by: Darrick J. Wong Signed-off-by: Darrick J. Wong