Re: [PATCH v5 0/8] Generic IRQ entry/exit support for powerpc

[Jirka Hladky]

Oh, I forgot to add the actual results from the LPAR:

Model name:                  POWER10 (architected), altivec supported
Model:                     2.0 (pvr 0080 0200)
Type                                         : Dedicated
Mode                                         : Capped
Entitled Capacity                            : 1.00
Memory Mode                                  : Dedicated

Results (median bogo-ops/sec, N=15 per cell):
  Mode          7.1        7.2-rc4      Delta
  Enforcing     68,275     70,154       +2.7%
  Permissive    68,082     67,166       -1.3%
  Disabled      99,654     100,510      +0.9%

On Thu, Sep 10, 2026 at 9:54 PM Jirka Hladky <jhladky@xxxxxxxxxx> wrote:
>
> Hi Mukesh,
>
> Following up on the ppc64le SELinux/kill() regression - it doesn't reproduce
> with proper multi-iteration sampling. stress-ng --kill has ~30%
> run-to-run variance
> on our LPARs. Part of the variance comes from LPARs being much less
> isolated than I'd assumed - we saw contention bleed across LPARs on the same
> physical server, even between "Dedicated" partitions. With 15 iterations per
> config, 7.1 and 7.2-rc4 are statistically even (within ~3%) across enforcing/
> permissive/disabled, confirmed on a second, physically separate LPAR (7.1 vs
> 7.2-rc6). This matches what you said - you couldn't reproduce it either.
>
> I also built 7.2-rc4 from source with your proposed
> rcu_ct_kernel_exit_qs() patch and re-ran the same sweep - it shows no
> consistent effect vs. stock 7.2-rc4 (differences within the same noise
> band), which is consistent with there being no real regression here for it
> to address.
>
> Sorry for the false alarm, and thank you for the patch and for taking the
> time to look into this - I don't think it's needed for this issue, though
> it may still be a reasonable efficiency improvement on its own merits (it
> does avoid the unconditional rcu_preempt_deferred_qs() call on the common
> fast path). I'll update the internal RHEL tracking ticket (RHEL-216740) to
> reflect that this doesn't reproduce.
>
> Thank you!
> Jirka
>
>
> On Tue, Sep 8, 2026 at 6:01 PM Jirka Hladky <jhladky@xxxxxxxxxx> wrote:
> >
> > Hi Mukesh,
> >
> > Re: policy details -  I'm running an ELN kernel on RHEL-10.2, with SELinux
> > in enforcing mode using the default (targeted) policy, nothing custom
> > or hardened on top of it. Let me know if you need anything more specific
> > (policy version, booleans, etc.) and I'll pull it.
> >
> > Re: the rcu_ct_kernel_exit_qs() diff — thanks, that guard against the
> > unconditional rcu_preempt_deferred_qs() call lines up well with what I
> > was seeing on the RCU-barrier side. I'll apply it on top of 7.2 on
> > POWER10 box and re-run the stress-ng --kill numbers
> > (enforcing/permissive/disabled) to see how much of the SELinux tax it
> > recovers. No need to prepare anything separately.
> >
> > I'm OOO tomorrow, but I'll get to this by the end of the week and report
> > back with results.
> >
> > Thanks
> > Jirka
> >
> >
> > On Tue, Sep 8, 2026 at 3:26 PM Mukesh Kumar Chaurasiya
> > <mkchauras@xxxxxxxxx> wrote:
> > >
> > > 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
> > > > <mkchauras@xxxxxxxxx> 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
> > >   *
> > >
> > >
> >
> >
> > --
> > -Jirka
>
>
>
> --
> -Jirka



-- 
-Jirka