• Michal Koutný's avatar
    cgroup: Optimize single thread migration · 9a3284fa
    Michal Koutný authored
    There are reports of users who use thread migrations between cgroups and
    they report performance drop after d59cfc09 ("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: default avatarMichal Koutný <mkoutny@suse.com>
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
    9a3284fa
cgroup-internal.h 8.65 KB