On Thu 02-07-20 09:37:38, Roman Gushchin wrote: > On Thu, Jul 02, 2020 at 06:22:02PM +0200, Michal Hocko wrote: > > On Wed 01-07-20 11:45:52, Roman Gushchin wrote: > > [...] > > > >From c97afecd32c0db5e024be9ba72f43d22974f5bcd Mon Sep 17 00:00:00 2001 > > > From: Roman Gushchin <guro@xxxxxx> > > > Date: Wed, 1 Jul 2020 11:05:32 -0700 > > > Subject: [PATCH] mm: kmem: make memcg_kmem_enabled() irreversible > > > > > > Historically the kernel memory accounting was an opt-in feature, which > > > could be enabled for individual cgroups. But now it's not true, and > > > it's on by default both on cgroup v1 and cgroup v2. And as long as a > > > user has at least one non-root memory cgroup, the kernel memory > > > accounting is on. So in most setups it's either always on (if memory > > > cgroups are in use and kmem accounting is not disabled), either always > > > off (otherwise). > > > > > > memcg_kmem_enabled() is used in many places to guard the kernel memory > > > accounting code. If memcg_kmem_enabled() can reverse from returning > > > true to returning false (as now), we can't rely on it on release paths > > > and have to check if it was on before. > > > > > > If we'll make memcg_kmem_enabled() irreversible (always returning true > > > after returning it for the first time), it'll make the general logic > > > more simple and robust. It also will allow to guard some checks which > > > otherwise would stay unguarded. > > > > > > Signed-off-by: Roman Gushchin <guro@xxxxxx> > > > --- > > > mm/memcontrol.c | 6 ++---- > > > 1 file changed, 2 insertions(+), 4 deletions(-) > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index 50ae77f3985e..2d018a51c941 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -3582,7 +3582,8 @@ static int memcg_online_kmem(struct mem_cgroup *memcg) > > > objcg->memcg = memcg; > > > rcu_assign_pointer(memcg->objcg, objcg); > > > > > > - static_branch_inc(&memcg_kmem_enabled_key); > > > + if (!memcg_kmem_enabled()) > > > + static_branch_inc(&memcg_kmem_enabled_key); > > > > Wouldn't be static_branch_enable() more readable? > > Agree, will change, add reported-by and tested-by tags and resend. > Thanks! Feel free to add Acked-by: Michal Hocko <mhocko@xxxxxxxx> > Btw, don't we wanna to change memcg_kmem_enabled() definition > from static_branch_unlikely() to static_branch_likely()? Honestly, I do not know what would be the impact but considering kmem is enabled unless explicitly disabled these days then likely sounds more logical from reading POV. I do not think that early allocations until the first memcg is created is the case to optimize for. Worth a separate patch I guess. -- Michal Hocko SUSE Labs
