| Message ID | 20210212170159.32153-3-songmuchun@bytedance.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [1/4] mm: memcontrol: remove memcg check from memcg_oom_recover | expand |
On Sat 13-02-21 01:01:58, Muchun Song wrote: > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > so idr_find() is pointless and wastes CPU cycles. Is this possible at all to happen? If not why should we add a test for _all_ invocations? > > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > --- > mm/memcontrol.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index a3f26522765a..68ed4b297c13 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > { > WARN_ON_ONCE(!rcu_read_lock_held()); > + /* The memcg ID cannot be zero. */ > + if (id == 0) > + return NULL; > return idr_find(&mem_cgroup_idr, id); > } > > -- > 2.11.0
On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko@suse.com> wrote: > > On Sat 13-02-21 01:01:58, Muchun Song wrote: > > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > > so idr_find() is pointless and wastes CPU cycles. > > Is this possible at all to happen? If not why should we add a test for > _all_ invocations? Yeah, this indeed can happen. If we allocate a new swap cache page and charge it via mem_cgroup_charge, then the page will uncharge the swap counter via mem_cgroup_uncharge_swap. When the swap entry is indeed freed, we will call mem_cgroup_uncharge_swap again, In this routine, we can pass zero to mem_cgroup_from_id. Right? > > > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > > --- > > mm/memcontrol.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > index a3f26522765a..68ed4b297c13 100644 > > --- a/mm/memcontrol.c > > +++ b/mm/memcontrol.c > > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > > { > > WARN_ON_ONCE(!rcu_read_lock_held()); > > + /* The memcg ID cannot be zero. */ > > + if (id == 0) > > + return NULL; > > return idr_find(&mem_cgroup_idr, id); > > } > > > > -- > > 2.11.0 > > -- > Michal Hocko > SUSE Labs
On Mon 15-02-21 18:09:44, Muchun Song wrote: > On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko@suse.com> wrote: > > > > On Sat 13-02-21 01:01:58, Muchun Song wrote: > > > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > > > so idr_find() is pointless and wastes CPU cycles. > > > > Is this possible at all to happen? If not why should we add a test for > > _all_ invocations? > > Yeah, this indeed can happen. If we allocate a new swap cache page > and charge it via mem_cgroup_charge, then the page will uncharge > the swap counter via mem_cgroup_uncharge_swap. When the swap > entry is indeed freed, we will call mem_cgroup_uncharge_swap again, > In this routine, we can pass zero to mem_cgroup_from_id. Right? If the above claim is correct, which I would need to double check then it should have been part of the changelog! Please think of your poor reviewers and the time they have to invest into the review. I would also like to see your waste of CPU cycles argument to be backed by something. Are we talking about cycles due to an additional function call? Is this really something we should even care about? > > > > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > > > --- > > > mm/memcontrol.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index a3f26522765a..68ed4b297c13 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > > > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > > > { > > > WARN_ON_ONCE(!rcu_read_lock_held()); > > > + /* The memcg ID cannot be zero. */ > > > + if (id == 0) > > > + return NULL; > > > return idr_find(&mem_cgroup_idr, id); > > > } > > > > > > -- > > > 2.11.0 > > > > -- > > Michal Hocko > > SUSE Labs
On Mon, Feb 15, 2021 at 6:27 PM Michal Hocko <mhocko@suse.com> wrote: > > On Mon 15-02-21 18:09:44, Muchun Song wrote: > > On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko@suse.com> wrote: > > > > > > On Sat 13-02-21 01:01:58, Muchun Song wrote: > > > > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > > > > so idr_find() is pointless and wastes CPU cycles. > > > > > > Is this possible at all to happen? If not why should we add a test for > > > _all_ invocations? > > > > Yeah, this indeed can happen. If we allocate a new swap cache page > > and charge it via mem_cgroup_charge, then the page will uncharge > > the swap counter via mem_cgroup_uncharge_swap. When the swap > > entry is indeed freed, we will call mem_cgroup_uncharge_swap again, > > In this routine, we can pass zero to mem_cgroup_from_id. Right? > > If the above claim is correct, which I would need to double check then > it should have been part of the changelog! Please think of your poor > reviewers and the time they have to invest into the review. The easy way may be adding a printk to mem_cgroup_from_id when the parameter is zero. > > I would also like to see your waste of CPU cycles argument to be backed > by something. Are we talking about cycles due to an additional function Yeah, when the parameter is already zero, idr_find() must return zero. So I think that the additional function call is unnecessary. I have added a printk to mem_cgroup_from_id, I found the parameter can be zero several times. > call? Is this really something we should even care about? Maybe not. Just my thoughts. Thanks. > > > > > > > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > > > > --- > > > > mm/memcontrol.c | 3 +++ > > > > 1 file changed, 3 insertions(+) > > > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > > index a3f26522765a..68ed4b297c13 100644 > > > > --- a/mm/memcontrol.c > > > > +++ b/mm/memcontrol.c > > > > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > > > > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > > > > { > > > > WARN_ON_ONCE(!rcu_read_lock_held()); > > > > + /* The memcg ID cannot be zero. */ > > > > + if (id == 0) > > > > + return NULL; > > > > return idr_find(&mem_cgroup_idr, id); > > > > } > > > > > > > > -- > > > > 2.11.0 > > > > > > -- > > > Michal Hocko > > > SUSE Labs > > -- > Michal Hocko > SUSE Labs
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index a3f26522765a..68ed4b297c13 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) struct mem_cgroup *mem_cgroup_from_id(unsigned short id) { WARN_ON_ONCE(!rcu_read_lock_held()); + /* The memcg ID cannot be zero. */ + if (id == 0) + return NULL; return idr_find(&mem_cgroup_idr, id); }
The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, so idr_find() is pointless and wastes CPU cycles. Signed-off-by: Muchun Song <songmuchun@bytedance.com> --- mm/memcontrol.c | 3 +++ 1 file changed, 3 insertions(+)