path: root/tools/testing/selftests/cgroup/cgroup_util.c
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authorMichal Koutný <mkoutny@suse.com>2019-10-04 12:57:40 +0200
committerTejun Heo <tj@kernel.org>2019-10-07 07:11:53 -0700
commit9a3284fad42f66bb43629c6716709ff791aaa457 (patch)
tree90ee5a200895f3be7b12fa1eff10fd1910afa019 /tools/testing/selftests/cgroup/cgroup_util.c
parentcgroup: Update comments about task exit path (diff)
cgroup: Optimize single thread migration
There are reports of users who use thread migrations between cgroups and they report performance drop after d59cfc09c32a ("sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem"). The effect is pronounced on machines with more CPUs. The migration is affected by forking noise happening in the background, after the mentioned commit a migrating thread must wait for all (forking) processes on the system, not only of its threadgroup. There are several places that need to synchronize with migration: a) do_exit, b) de_thread, c) copy_process, d) cgroup_update_dfl_csses, e) parallel migration (cgroup_{proc,thread}s_write). In the case of self-migrating thread, we relax the synchronization on cgroup_threadgroup_rwsem to avoid the cost of waiting. d) and e) are excluded with cgroup_mutex, c) does not matter in case of single thread migration and the executing thread cannot exec(2) or exit(2) while it is writing into cgroup.threads. In case of do_exit because of signal delivery, we either exit before the migration or finish the migration (of not yet PF_EXITING thread) and die afterwards. This patch handles only the case of self-migration by writing "0" into cgroup.threads. For simplicity, we always take cgroup_threadgroup_rwsem with numeric PIDs. This change improves migration dependent workload performance similar to per-signal_struct state. Signed-off-by: Michal Koutný <mkoutny@suse.com> Signed-off-by: Tejun Heo <tj@kernel.org>
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