| Message ID | 20230830175335.1536008-5-yosryahmed@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | memcg: non-unified flushing for userspace stats | expand |
Hi Yosry, kernel test robot noticed the following build warnings: [auto build test WARNING on akpm-mm/mm-everything] [also build test WARNING on linus/master next-20230830] [cannot apply to v6.5] [If your patch is applied to the wrong git tree, kindly drop us a note. And when submitting patch, we suggest to use '--base' as documented in https://git-scm.com/docs/git-format-patch#_base_tree_information] url: https://github.com/intel-lab-lkp/linux/commits/Yosry-Ahmed/mm-memcg-properly-name-and-document-unified-stats-flushing/20230831-015518 base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything patch link: https://lore.kernel.org/r/20230830175335.1536008-5-yosryahmed%40google.com patch subject: [PATCH v3 4/4] mm: memcg: use non-unified stats flushing for userspace reads config: m68k-randconfig-r016-20230831 (https://download.01.org/0day-ci/archive/20230831/202308310858.19VshX68-lkp@intel.com/config) compiler: m68k-linux-gcc (GCC) 13.2.0 reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230831/202308310858.19VshX68-lkp@intel.com/reproduce) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp@intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202308310858.19VshX68-lkp@intel.com/ All warnings (new ones prefixed by >>): >> mm/memcontrol.c:667:6: warning: no previous prototype for 'mem_cgroup_user_flush_stats' [-Wmissing-prototypes] 667 | void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) | ^~~~~~~~~~~~~~~~~~~~~~~~~~~ vim +/mem_cgroup_user_flush_stats +667 mm/memcontrol.c 658 659 /* 660 * mem_cgroup_user_flush_stats - do a stats flush for a user read 661 * @memcg: memory cgroup to flush 662 * 663 * Flush the subtree of @memcg. A mutex is used for userspace readers to gate 664 * the global rstat spinlock. This protects in-kernel flushers from userspace 665 * readers hogging the lock. 666 */ > 667 void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) 668 { 669 mutex_lock(&stats_user_flush_mutex); 670 do_stats_flush(memcg); 671 mutex_unlock(&stats_user_flush_mutex); 672 } 673
Hi Yosry, kernel test robot noticed the following build warnings: [auto build test WARNING on akpm-mm/mm-everything] [also build test WARNING on linus/master next-20230830] [cannot apply to v6.5] [If your patch is applied to the wrong git tree, kindly drop us a note. And when submitting patch, we suggest to use '--base' as documented in https://git-scm.com/docs/git-format-patch#_base_tree_information] url: https://github.com/intel-lab-lkp/linux/commits/Yosry-Ahmed/mm-memcg-properly-name-and-document-unified-stats-flushing/20230831-015518 base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything patch link: https://lore.kernel.org/r/20230830175335.1536008-5-yosryahmed%40google.com patch subject: [PATCH v3 4/4] mm: memcg: use non-unified stats flushing for userspace reads config: i386-randconfig-r013-20230831 (https://download.01.org/0day-ci/archive/20230831/202308311025.538QuXBV-lkp@intel.com/config) compiler: clang version 16.0.4 (https://github.com/llvm/llvm-project.git ae42196bc493ffe877a7e3dff8be32035dea4d07) reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230831/202308311025.538QuXBV-lkp@intel.com/reproduce) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp@intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202308311025.538QuXBV-lkp@intel.com/ All warnings (new ones prefixed by >>): >> mm/memcontrol.c:667:6: warning: no previous prototype for function 'mem_cgroup_user_flush_stats' [-Wmissing-prototypes] void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) ^ mm/memcontrol.c:667:1: note: declare 'static' if the function is not intended to be used outside of this translation unit void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) ^ static 1 warning generated. vim +/mem_cgroup_user_flush_stats +667 mm/memcontrol.c 658 659 /* 660 * mem_cgroup_user_flush_stats - do a stats flush for a user read 661 * @memcg: memory cgroup to flush 662 * 663 * Flush the subtree of @memcg. A mutex is used for userspace readers to gate 664 * the global rstat spinlock. This protects in-kernel flushers from userspace 665 * readers hogging the lock. 666 */ > 667 void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) 668 { 669 mutex_lock(&stats_user_flush_mutex); 670 do_stats_flush(memcg); 671 mutex_unlock(&stats_user_flush_mutex); 672 } 673
On Wed, Aug 30, 2023 at 8:08 PM kernel test robot <lkp@intel.com> wrote: > > Hi Yosry, > > kernel test robot noticed the following build warnings: > > [auto build test WARNING on akpm-mm/mm-everything] > [also build test WARNING on linus/master next-20230830] > [cannot apply to v6.5] > [If your patch is applied to the wrong git tree, kindly drop us a note. > And when submitting patch, we suggest to use '--base' as documented in > https://git-scm.com/docs/git-format-patch#_base_tree_information] > > url: https://github.com/intel-lab-lkp/linux/commits/Yosry-Ahmed/mm-memcg-properly-name-and-document-unified-stats-flushing/20230831-015518 > base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything > patch link: https://lore.kernel.org/r/20230830175335.1536008-5-yosryahmed%40google.com > patch subject: [PATCH v3 4/4] mm: memcg: use non-unified stats flushing for userspace reads > config: i386-randconfig-r013-20230831 (https://download.01.org/0day-ci/archive/20230831/202308311025.538QuXBV-lkp@intel.com/config) > compiler: clang version 16.0.4 (https://github.com/llvm/llvm-project.git ae42196bc493ffe877a7e3dff8be32035dea4d07) > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230831/202308311025.538QuXBV-lkp@intel.com/reproduce) > > If you fix the issue in a separate patch/commit (i.e. not just a new version of > the same patch/commit), kindly add following tags > | Reported-by: kernel test robot <lkp@intel.com> > | Closes: https://lore.kernel.org/oe-kbuild-all/202308311025.538QuXBV-lkp@intel.com/ > > All warnings (new ones prefixed by >>): > > >> mm/memcontrol.c:667:6: warning: no previous prototype for function 'mem_cgroup_user_flush_stats' [-Wmissing-prototypes] > void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) > ^ > mm/memcontrol.c:667:1: note: declare 'static' if the function is not intended to be used outside of this translation unit > void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) > ^ > static > 1 warning generated. Ah silly mistake on my end. Will send v4 after making mem_cgroup_user_flush_stats() static. > > > vim +/mem_cgroup_user_flush_stats +667 mm/memcontrol.c > > 658 > 659 /* > 660 * mem_cgroup_user_flush_stats - do a stats flush for a user read > 661 * @memcg: memory cgroup to flush > 662 * > 663 * Flush the subtree of @memcg. A mutex is used for userspace readers to gate > 664 * the global rstat spinlock. This protects in-kernel flushers from userspace > 665 * readers hogging the lock. > 666 */ > > 667 void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) > 668 { > 669 mutex_lock(&stats_user_flush_mutex); > 670 do_stats_flush(memcg); > 671 mutex_unlock(&stats_user_flush_mutex); > 672 } > 673 > > -- > 0-DAY CI Kernel Test Service > https://github.com/intel/lkp-tests/wiki
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 94d5a6751a9e..1544c3964f19 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -588,6 +588,7 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_node *mctz) static void flush_memcg_stats_dwork(struct work_struct *w); static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork); static DEFINE_PER_CPU(unsigned int, stats_updates); +static DEFINE_MUTEX(stats_user_flush_mutex); static atomic_t stats_unified_flush_ongoing = ATOMIC_INIT(0); static atomic_t stats_flush_threshold = ATOMIC_INIT(0); static u64 flush_next_time; @@ -655,6 +656,21 @@ static void do_stats_flush(struct mem_cgroup *memcg) cgroup_rstat_flush(memcg->css.cgroup); } +/* + * mem_cgroup_user_flush_stats - do a stats flush for a user read + * @memcg: memory cgroup to flush + * + * Flush the subtree of @memcg. A mutex is used for userspace readers to gate + * the global rstat spinlock. This protects in-kernel flushers from userspace + * readers hogging the lock. + */ +void mem_cgroup_user_flush_stats(struct mem_cgroup *memcg) +{ + mutex_lock(&stats_user_flush_mutex); + do_stats_flush(memcg); + mutex_unlock(&stats_user_flush_mutex); +} + /* * do_unified_stats_flush - do a unified flush of memory cgroup statistics * @@ -1608,7 +1624,7 @@ static void memcg_stat_format(struct mem_cgroup *memcg, struct seq_buf *s) * * Current memory state: */ - mem_cgroup_try_flush_stats(); + mem_cgroup_user_flush_stats(memcg); for (i = 0; i < ARRAY_SIZE(memory_stats); i++) { u64 size; @@ -4050,7 +4066,7 @@ static int memcg_numa_stat_show(struct seq_file *m, void *v) int nid; struct mem_cgroup *memcg = mem_cgroup_from_seq(m); - mem_cgroup_try_flush_stats(); + mem_cgroup_user_flush_stats(memcg); for (stat = stats; stat < stats + ARRAY_SIZE(stats); stat++) { seq_printf(m, "%s=%lu", stat->name, @@ -4125,7 +4141,7 @@ static void memcg1_stat_format(struct mem_cgroup *memcg, struct seq_buf *s) BUILD_BUG_ON(ARRAY_SIZE(memcg1_stat_names) != ARRAY_SIZE(memcg1_stats)); - mem_cgroup_try_flush_stats(); + mem_cgroup_user_flush_stats(memcg); for (i = 0; i < ARRAY_SIZE(memcg1_stats); i++) { unsigned long nr; @@ -6642,7 +6658,7 @@ static int memory_numa_stat_show(struct seq_file *m, void *v) int i; struct mem_cgroup *memcg = mem_cgroup_from_seq(m); - mem_cgroup_try_flush_stats(); + mem_cgroup_user_flush_stats(memcg); for (i = 0; i < ARRAY_SIZE(memory_stats); i++) { int nid;
Unified flushing allows for great concurrency for paths that attempt to flush the stats, at the expense of potential staleness and a single flusher paying the extra cost of flushing the full tree. This tradeoff makes sense for in-kernel flushers that may observe high concurrency (e.g. reclaim, refault). For userspace readers, stale stats may be unexpected and problematic, especially when such stats are used for critical paths such as userspace OOM handling. Additionally, a userspace reader will occasionally pay the cost of flushing the entire hierarchy, which also causes problems in some cases [1]. Opt userspace reads out of unified flushing. This makes the cost of reading the stats more predictable (proportional to the size of the subtree), as well as the freshness of the stats. Userspace readers are not expected to have similar concurrency to in-kernel flushers, serializing them among themselves and among in-kernel flushers should be okay. Nonetheless, for extra safety, introduce a mutex when flushing for userspace readers to make sure only a single userspace reader can compete with in-kernel flushers at a time. This takes away userspace ability to directly influence or hurt in-kernel lock contention. An alternative is to remove flushing from the stats reading path completely, and rely on the periodic flusher. This should be accompanied by making the periodic flushing period tunable, and providing an interface for userspace to force a flush, following a similar model to /proc/vmstat. However, such a change will be hard to reverse if the implementation needs to be changed because: - The cost of reading stats will be very cheap and we won't be able to take that back easily. - There are user-visible interfaces involved. Hence, let's go with the change that's most reversible first and revisit as needed. This was tested on a machine with 256 cpus by running a synthetic test script [2] that creates 50 top-level cgroups, each with 5 children (250 leaf cgroups). Each leaf cgroup has 10 processes running that allocate memory beyond the cgroup limit, invoking reclaim (which is an in-kernel unified flusher). Concurrently, one thread is spawned per-cgroup to read the stats every second (including root, top-level, and leaf cgroups -- so total 251 threads). No significant regressions were observed in the total run time, which means that userspace readers are not significantly affecting in-kernel flushers: Base (mm-unstable): real 0m22.500s user 0m9.399s sys 73m41.381s real 0m22.749s user 0m15.648s sys 73m13.113s real 0m22.466s user 0m10.000s sys 73m11.933s With this patch: real 0m23.092s user 0m10.110s sys 75m42.774s real 0m22.277s user 0m10.443s sys 72m7.182s real 0m24.127s user 0m12.617s sys 78m52.765s [1]https://lore.kernel.org/lkml/CABWYdi0c6__rh-K7dcM_pkf9BJdTRtAU08M43KO9ME4-dsgfoQ@mail.gmail.com/ [2]https://lore.kernel.org/lkml/CAJD7tka13M-zVZTyQJYL1iUAYvuQ1fcHbCjcOBZcz6POYTV-4g@mail.gmail.com/ Signed-off-by: Yosry Ahmed <yosryahmed@google.com> --- mm/memcontrol.c | 24 ++++++++++++++++++++---- 1 file changed, 20 insertions(+), 4 deletions(-)