Re: [tip:timers/core] [posix] 1535cb8028: stress-ng.epoll.ops_per_sec 36.2% regression

From: Eric Dumazet
Date: Thu Mar 27 2025 - 07:38:07 EST


On Thu, Mar 27, 2025 at 11:50 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Thu, Mar 27 2025 at 10:11, Eric Dumazet wrote:
> > On Thu, Mar 27, 2025 at 9:26 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> >> We could place all these atomic fields in separate cache lines,
> >> to keep read-only fields shared as much as possible.
> >>
> >
> > Following one-liner seems good enough to separate the 4 atomics used
> > to control/limit
> >
> > UCOUNT_RLIMIT_NPROC, UCOUNT_RLIMIT_MSGQUEUE, UCOUNT_RLIMIT_SIGPENDING,
> > UCOUNT_RLIMIT_MEMLOCK,
> >
> >
> > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> > index 7183e5aca282..6cc3fbec3632 100644
> > --- a/include/linux/user_namespace.h
> > +++ b/include/linux/user_namespace.h
> > @@ -120,7 +120,7 @@ struct ucounts {
> > kuid_t uid;
> > atomic_t count;
> > atomic_long_t ucount[UCOUNT_COUNTS];
> > - atomic_long_t rlimit[UCOUNT_RLIMIT_COUNTS];
> > + atomic_long_t ____cacheline_aligned_in_smp rlimit[UCOUNT_RLIMIT_COUNTS];
> > };
>
> Cute. How much bloat does it cause?

This would expand 'struct ucounts' by 192 bytes on x86, if the patch
was actually working :)

Note sure if it is feasible without something more intrusive like

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 7183e5aca282..3513df720430 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -120,7 +120,10 @@ struct ucounts {
kuid_t uid;
atomic_t count;
atomic_long_t ucount[UCOUNT_COUNTS];
- atomic_long_t rlimit[UCOUNT_RLIMIT_COUNTS];
+
+ struct {
+ atomic_long_t ____cacheline_aligned_in_smp val;
+ } rlimit[UCOUNT_RLIMIT_COUNTS];
};

extern struct user_namespace init_user_ns;
@@ -136,7 +139,7 @@ void put_ucounts(struct ucounts *ucounts);

static inline long get_rlimit_value(struct ucounts *ucounts, enum
rlimit_type type)
{
- return atomic_long_read(&ucounts->rlimit[type]);
+ return atomic_long_read(&ucounts->rlimit[type].val);
}

long inc_rlimit_ucounts(struct ucounts *ucounts, enum rlimit_type
type, long v);
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 86c5f1c0bad9..0cd5498890f8 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -266,7 +266,7 @@ long inc_rlimit_ucounts(struct ucounts *ucounts,
enum rlimit_type type, long v)
long ret = 0;

for (iter = ucounts; iter; iter = iter->ns->ucounts) {
- long new = atomic_long_add_return(v, &iter->rlimit[type]);
+ long new = atomic_long_add_return(v, &iter->rlimit[type].val);
if (new < 0 || new > max)
ret = LONG_MAX;
else if (iter == ucounts)
@@ -281,7 +281,7 @@ bool dec_rlimit_ucounts(struct ucounts *ucounts,
enum rlimit_type type, long v)
struct ucounts *iter;
long new = -1; /* Silence compiler warning */
for (iter = ucounts; iter; iter = iter->ns->ucounts) {
- long dec = atomic_long_sub_return(v, &iter->rlimit[type]);
+ long dec = atomic_long_sub_return(v, &iter->rlimit[type].val);
WARN_ON_ONCE(dec < 0);
if (iter == ucounts)
new = dec;
@@ -294,7 +294,7 @@ static void do_dec_rlimit_put_ucounts(struct
ucounts *ucounts,
{
struct ucounts *iter, *next;
for (iter = ucounts; iter != last; iter = next) {
- long dec = atomic_long_sub_return(1, &iter->rlimit[type]);
+ long dec = atomic_long_sub_return(1, &iter->rlimit[type].val);
WARN_ON_ONCE(dec < 0);
next = iter->ns->ucounts;
if (dec == 0)
@@ -316,7 +316,7 @@ long inc_rlimit_get_ucounts(struct ucounts
*ucounts, enum rlimit_type type,
long dec, ret = 0;

for (iter = ucounts; iter; iter = iter->ns->ucounts) {
- long new = atomic_long_add_return(1, &iter->rlimit[type]);
+ long new = atomic_long_add_return(1, &iter->rlimit[type].val);
if (new < 0 || new > max)
goto dec_unwind;
if (iter == ucounts)
@@ -334,7 +334,7 @@ long inc_rlimit_get_ucounts(struct ucounts
*ucounts, enum rlimit_type type,
}
return ret;
dec_unwind:
- dec = atomic_long_sub_return(1, &iter->rlimit[type]);
+ dec = atomic_long_sub_return(1, &iter->rlimit[type].val);
WARN_ON_ONCE(dec < 0);
do_dec_rlimit_put_ucounts(ucounts, iter, type);
return 0;