From ef62c8ff1de437ec2e0582205c1293c7dbbc4484 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Thu, 24 Mar 2022 18:09:02 -0700 Subject: lib/vsprintf: avoid redundant work with 0 size Patch series "mm/page_owner: Extend page_owner to show memcg information", v4. While debugging the constant increase in percpu memory consumption on a system that spawned large number of containers, it was found that a lot of offline mem_cgroup structures remained in place without being freed. Further investigation indicated that those mem_cgroup structures were pinned by some pages. In order to find out what those pages are, the existing page_owner debugging tool is extended to show memory cgroup information and whether those memcgs are offline or not. With the enhanced page_owner tool, the following is a typical page that pinned the mem_cgroup structure in my test case: Page allocated via order 0, mask 0x1100cca(GFP_HIGHUSER_MOVABLE), pid 162970 (podman), ts 1097761405537 ns, free_ts 1097760838089 ns PFN 1925700 type Movable Block 3761 type Movable Flags 0x17ffffc00c001c(uptodate|dirty|lru|reclaim|swapbacked|node=0|zone=2|lastcpupid=0x1fffff) prep_new_page+0xac/0xe0 get_page_from_freelist+0x1327/0x14d0 __alloc_pages+0x191/0x340 alloc_pages_vma+0x84/0x250 shmem_alloc_page+0x3f/0x90 shmem_alloc_and_acct_page+0x76/0x1c0 shmem_getpage_gfp+0x281/0x940 shmem_write_begin+0x36/0xe0 generic_perform_write+0xed/0x1d0 __generic_file_write_iter+0xdc/0x1b0 generic_file_write_iter+0x5d/0xb0 new_sync_write+0x11f/0x1b0 vfs_write+0x1ba/0x2a0 ksys_write+0x59/0xd0 do_syscall_64+0x37/0x80 entry_SYSCALL_64_after_hwframe+0x44/0xae Charged to offline memcg libpod-conmon-15e4f9c758422306b73b2dd99f9d50a5ea53cbb16b4a13a2c2308a4253cc0ec8. So the page was not freed because it was part of a shmem segment. That is useful information that can help users to diagnose similar problems. With cgroup v1, /proc/cgroups can be read to find out the total number of memory cgroups (online + offline). With cgroup v2, the cgroup.stat of the root cgroup can be read to find the number of dying cgroups (most likely pinned by dying memcgs). The page_owner feature is not supposed to be enabled for production system due to its memory overhead. However, if it is suspected that dying memcgs are increasing over time, a test environment with page_owner enabled can then be set up with appropriate workload for further analysis on what may be causing the increasing number of dying memcgs. This patch (of 4): For *scnprintf(), vsnprintf() is always called even if the input size is 0. That is a waste of time, so just return 0 in this case. Note that vsnprintf() will never return -1 to indicate an error. So skipping the call to vsnprintf() when size is 0 will have no functional impact at all. Link: https://lkml.kernel.org/r/20220202203036.744010-1-longman@redhat.com Link: https://lkml.kernel.org/r/20220202203036.744010-2-longman@redhat.com Signed-off-by: Waiman Long Acked-by: David Rientjes Reviewed-by: Sergey Senozhatsky Acked-by: Roman Gushchin Acked-by: Rafael Aquini Acked-by: Mike Rapoport Cc: Roman Gushchin Cc: Johannes Weiner Cc: Michal Hocko Cc: Vladimir Davydov Cc: Petr Mladek Cc: Steven Rostedt (Google) Cc: Andy Shevchenko Cc: Rasmus Villemoes Cc: Ira Weiny Cc: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- lib/vsprintf.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) (limited to 'lib') diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 53fe73a48adf..40d26a07a133 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -2906,13 +2906,15 @@ int vscnprintf(char *buf, size_t size, const char *fmt, va_list args) { int i; + if (unlikely(!size)) + return 0; + i = vsnprintf(buf, size, fmt, args); if (likely(i < size)) return i; - if (size != 0) - return size - 1; - return 0; + + return size - 1; } EXPORT_SYMBOL(vscnprintf); -- cgit v1.2.3-58-ga151