On Thu, Jul 30, 2026 at 01:24:46PM +0200, Jirka Hladky wrote:
> On Thu, Jul 30, 2026 at 11:36 AM Mukesh Kumar Chaurasiya
> <[email protected]> wrote:
> >
> > On Mon, Jul 27, 2026 at 11:41:07AM +0200, Jirka Hladky wrote:
> > > Hi Mukesh, Shrikanth, Madhavan,
> > >
> > > I'm seeing a ~19% throughput regression on ppc64le (POWER10) in
> > > syscall-heavy workloads between kernel 7.1 and 7.2-rc4, which I've
> > > narrowed down to the GENERIC_ENTRY enablement for powerpc (commit
> > > bee25f97ad24 "powerpc: Enable GENERIC_ENTRY feature").
> > >
> > > The regression manifests specifically when SELinux is enabled. Regression
> > > disappears with selinux=0. With SELinux disabled, 7.2 is actually
> > > slightly *faster* than 7.1.
> > >
> > > Test: stress-ng --kill 1 -t 23 (single-thread kill() syscall loop)
> > > Machine: IBM,9080-HEX POWER10, 8 vCPUs (1 core SMT-8), 64 GiB
> > >
> > > Results (bogo-ops/sec, higher is better):
> > >
> > > Kernel Enforcing Permissive Disabled SELinux tax
> > > ------ --------- ---------- -------- -----------
> > > 7.1 69,107 70,248 93,566 -26.1%
> > > 7.2-rc4 56,247 59,075 99,240 -43.3%
> > > Delta -18.6% -15.9% +6.1%
> > >
> > > Key observations:
> > > - SELinux permissive ~ enforcing on both kernels (overhead is in the
> > > code path, not policy evaluation)
> > > - SELinux disabled: 7.2 is 6% faster than 7.1 -- the generic entry
> > > path itself is not slower for the base syscall
> > > - The SELinux "tax" nearly doubles from 26% to 43% between 7.1->7.2
> > > - No changes to security/selinux/avc.c between v7.1 and v7.2-rc4
> > >
> > > The kill() hot path (sys_kill -> check_kill_permission ->
> > > security_task_kill
> > > -> selinux_task_kill -> avc_has_perm -> avc_lookup) wraps every AVC lookup
> > > in rcu_read_lock()/rcu_read_unlock(). On ppc64le with PREEMPT_RCU, each
> > > pair requires lwsync/isync barriers. I suspect the generic entry path
> > > changes something in how context tracking, tracing, or RCU interacts
> > > with the syscall fast path that amplifies the per-call RCU cost.
> > >
> > > Note: this is on top of a separate ~36% regression from 6.12->7.1 caused
> > > by HAVE_PREEMPT_DYNAMIC_KEY enabling CONFIG_PREEMPT_RCU on ppc64le
> > > (commit 6ad7751537e8), which I reported separately on the v4 thread.
> > >
> > > Thank you
> > > Jirka
> > [...]
> > Hey Jirka,
> >
> > Thanks for reporting this.
> > I'll take a look.
> >
> > Thanks,
> > Mukesh
> >
>
> Thank you, Mukesh! I'm happy to run additional tests on the POWER10 machine
> if you have patches to try or specific experiments you'd like me to do.
>
> Jirka
>
can you try this with the below diff?
diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index e56ded733b1b..dcad641eb2c2 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -120,6 +120,7 @@ static inline bool rcu_preempt_need_deferred_qs(struct
task_struct *t)
return false;
}
static inline void rcu_preempt_deferred_qs(struct task_struct *t) { }
+static inline void rcu_ct_kernel_exit_qs(void) { }
void rcu_scheduler_starting(void);
static inline void rcu_end_inkernel_boot(void) { }
static inline bool rcu_inkernel_boot_has_ended(void) { return true; }
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 16a04202888b..d623f2a7d3fc 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -87,6 +87,7 @@ static inline void rcu_irq_exit_check_preempt(void) { }
struct task_struct;
void rcu_preempt_deferred_qs(struct task_struct *t);
+void rcu_ct_kernel_exit_qs(void);
void exit_rcu(void);
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index a743e7ffa6c0..011018214c6d 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -118,7 +118,7 @@ static void noinstr ct_kernel_exit(bool user, int offset)
lockdep_assert_irqs_disabled();
trace_rcu_watching(TPS("End"), ct_nesting(), 0, ct_rcu_watching());
WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user &&
!is_idle_task(current));
- rcu_preempt_deferred_qs(current);
+ rcu_ct_kernel_exit_qs();
// instrumentation for the noinstr ct_kernel_exit_state()
instrument_atomic_write(&ct->state, sizeof(ct->state));
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 96848fc1f02b..c23478f70c17 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -368,6 +368,25 @@ notrace void rcu_momentary_eqs(void)
}
EXPORT_SYMBOL_GPL(rcu_momentary_eqs);
+/**
+ * rcu_ct_kernel_exit_qs - report deferred QS on syscall/exception exit if
needed
+ *
+ * Called from ct_kernel_exit() on every return to userspace. Guards the
+ * rcu_preempt_deferred_qs() call with rcu_preempt_need_deferred_qs() so that
+ * on the common fast path -- where nothing is deferred -- we avoid the
+ * cache-line traffic on current->rcu_read_unlock_special that the
unconditional
+ * call causes. This is particularly significant on weakly-ordered
architectures
+ * (e.g. ppc64le) where rcu_read_lock/unlock issue lwsync/isync barriers and
+ * already touch that cache line in the syscall body.
+ *
+ * Follows the same pattern used by rcu_flavor_sched_clock_irq().
+ */
+notrace void rcu_ct_kernel_exit_qs(void)
+{
+ if (rcu_preempt_need_deferred_qs(current))
+ rcu_preempt_deferred_qs(current);
+}
+
/**
* rcu_is_cpu_rrupt_from_idle - see if 'interrupted' from idle
*
