From 20e43cd69485c683f2b7cf473cc93c216fc59d03 Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Tue, 27 Oct 2020 14:35:35 +0100 Subject: [PATCH] proc.5: Document inaccurate RSS due to SPLIT_RSS_COUNTING [mtk: Manually applied patch, because of conflicts with other merged changes; also added an edit suggested by Jann; see the thread at https://lore.kernel.org/linux-man/20201012114940.1317510-1-jannh@google.com/] Since 34e55232e59f7b19050267a05ff1226e5cd122a5 (introduced back in v2.6.34), Linux uses per-thread RSS counters to reduce cache contention on the per-mm counters. With a 4K page size, that means that you can end up with the counters off by up to 252KiB per thread. Example: $ cat rsstest.c #include #include #include #include #include #include #include #include void dump(int pid) { char cmd[1000]; sprintf(cmd, "grep '^VmRSS' /proc/%d/status;" "grep '^Rss:' /proc/%d/smaps_rollup;" "echo", pid, pid ); system(cmd); } int main(void) { eventfd_t dummy; int child_wait = eventfd(0, EFD_SEMAPHORE|EFD_CLOEXEC); int child_resume = eventfd(0, EFD_SEMAPHORE|EFD_CLOEXEC); if (child_wait == -1 || child_resume == -1) err(1, "eventfd"); pid_t child = fork(); if (child == -1) err(1, "fork"); if (child == 0) { if (prctl(PR_SET_PDEATHSIG, SIGKILL)) err(1, "PDEATHSIG"); if (getppid() == 1) exit(0); char *mapping = mmap(NULL, 80 * 0x1000, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); eventfd_write(child_wait, 1); eventfd_read(child_resume, &dummy); for (int i=0; i<40; i++) mapping[0x1000 * i] = 1; eventfd_write(child_wait, 1); eventfd_read(child_resume, &dummy); for (int i=40; i<80; i++) mapping[0x1000 * i] = 1; eventfd_write(child_wait, 1); eventfd_read(child_resume, &dummy); exit(0); } eventfd_read(child_wait, &dummy); dump(child); eventfd_write(child_resume, 1); eventfd_read(child_wait, &dummy); dump(child); eventfd_write(child_resume, 1); eventfd_read(child_wait, &dummy); dump(child); eventfd_write(child_resume, 1); exit(0); } $ gcc -o rsstest rsstest.c && ./rsstest VmRSS: 68 kB Rss: 616 kB VmRSS: 68 kB Rss: 776 kB VmRSS: 812 kB Rss: 936 kB $ Let's document that those counters aren't entirely accurate. Reported-by: Mark Mossberg Signed-off-by: Jann Horn Signed-off-by: Michael Kerrisk --- man5/proc.5 | 36 ++++++++++++++++++++++++++++++++++-- 1 file changed, 34 insertions(+), 2 deletions(-) diff --git a/man5/proc.5 b/man5/proc.5 index 213fc004e..48d0b5e06 100644 --- a/man5/proc.5 +++ b/man5/proc.5 @@ -2265,6 +2265,9 @@ This is just the pages which count toward text, data, or stack space. This does not include pages which have not been demand-loaded in, or which are swapped out. +This value is inaccurate; see +.I /proc/[pid]/statm +below. .TP (25) \fIrsslim\fP \ %lu Current soft limit in bytes on the rss of the process; @@ -2409,10 +2412,11 @@ The columns are: size (1) total program size (same as VmSize in \fI/proc/[pid]/status\fP) resident (2) resident set size - (same as VmRSS in \fI/proc/[pid]/status\fP) + (inaccurate; same as VmRSS in \fI/proc/[pid]/status\fP) shared (3) number of resident shared pages (i.e., backed by a file) - (same as RssFile+RssShmem in \fI/proc/[pid]/status\fP) + (inaccurate; same as RssFile+RssShmem in + \fI/proc/[pid]/status\fP) text (4) text (code) .\" (not including libs; broken, includes data segment) lib (5) library (unused since Linux 2.6; always 0) @@ -2421,6 +2425,16 @@ data (6) data + stack dt (7) dirty pages (unused since Linux 2.6; always 0) .EE .in +.IP +.\" See SPLIT_RSS_COUNTING in the kernel. +.\" Inaccuracy is bounded by TASK_RSS_EVENTS_THRESH. +Some of these values are inaccurate because +of a kernel-internal scalability optimization. +If accurate values are required, use +.I /proc/[pid]/smaps +or +.I /proc/[pid]/smaps_rollup +instead, which are much slower but provide accurate, detailed information. .TP .I /proc/[pid]/status Provides much of the information in @@ -2597,6 +2611,9 @@ directly access physical memory. .TP .IR VmHWM Peak resident set size ("high water mark"). +This value is inaccurate; see +.I /proc/[pid]/statm +above. .TP .IR VmRSS Resident set size. @@ -2605,16 +2622,25 @@ Note that the value here is the sum of .IR RssFile , and .IR RssShmem . +This value is inaccurate; see +.I /proc/[pid]/statm +above. .TP .IR RssAnon Size of resident anonymous memory. .\" commit bf9683d6990589390b5178dafe8fd06808869293 (since Linux 4.5). +This value is inaccurate; see +.I /proc/[pid]/statm +above. .TP .IR RssFile Size of resident file mappings. .\" commit bf9683d6990589390b5178dafe8fd06808869293 (since Linux 4.5). +This value is inaccurate; see +.I /proc/[pid]/statm +above. .TP .IR RssShmem Size of resident shared memory (includes System V shared memory, @@ -2626,6 +2652,9 @@ and shared anonymous mappings). .TP .IR VmData ", " VmStk ", " VmExe Size of data, stack, and text segments. +This value is inaccurate; see +.I /proc/[pid]/statm +above. .TP .IR VmLib Shared library code size. @@ -2641,6 +2670,9 @@ Size of second-level page tables (added in Linux 4.0; removed in Linux 4.15). .\" commit b084d4353ff99d824d3bc5a5c2c22c70b1fba722 Swapped-out virtual memory size by anonymous private pages; shmem swap usage is not included (since Linux 2.6.34). +This value is inaccurate; see +.I /proc/[pid]/statm +above. .TP .IR HugetlbPages Size of hugetlb memory portions