Re: [PATCH v4 1/1] powerpc: enable dynamic preemption

[Christophe Leroy (CS GROUP)]

Hi Shrikanth,

Le 27/07/2026 à 12:28, Shrikanth Hegde a écrit :
> Hi Jirka,
> Please avoid top-posting.
>
> On 7/27/26 3:43 PM, Jirka Hladky wrote:
> > Hi Shrikanth,
> >
> > Thanks for the quick response and for looping in Paul.
> >
> > > This is true only if user selected PREEMPT_DYNAMIC option i think.
> >
> > Yes, exactly. The issue is that CONFIG_PREEMPT_DYNAMIC=y has been in
> > the Fedora/RHEL kernel config since kernel 5.16 (2021). It was
> > silently ignored on ppc64le until your 6.16 patch added
> > HAVE_PREEMPT_DYNAMIC_KEY, so this is the first time PREEMPT_RCU
> > actually takes effect for all Fedora/RHEL ppc64le users.
> >
> > > Does your preemption mode remain the same in two cases?
> >
> > Verified. On 6.15-rc6 (before PREEMPT_DYNAMIC takes effect):
> >
> >    CONFIG_PREEMPT_VOLUNTARY=y
> >    (no CONFIG_PREEMPT_DYNAMIC, no /sys/kernel/debug/sched/preempt)
> >
> > On both 7.1 and 7.2-rc4 (with PREEMPT_DYNAMIC active):
> >
> >    CONFIG_PREEMPT_LAZY=y
> >    CONFIG_PREEMPT_DYNAMIC=y
> >    CONFIG_PREEMPT_RCU=y
> >    cat /sys/kernel/debug/sched/preempt: "full (lazy)"
> >
> > So the runtime preemption mode is the same on 7.1 and 7.2. The
> > difference between 6.15 and 7.1+ is that the preemption model changed
> > from voluntary (static) to full/lazy (dynamic), which is what pulls
> > in PREEMPT_RCU.
>
> That means comparison is between preempt=voluntary vs preempt=lazy.

As far as I can see CONFIG_PREEMPT_LAZY doesn't pull CONFIG_PREEMPT_RCU. 
CONFIG_PREEMPT_DYNAMIC does:

config PREEMPT_RCU
	bool
	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
	select TREE_RCU
	help
	  This option selects the RCU implementation that is
	  designed for very large SMP systems with hundreds or
	  thousands of CPUs, but for which real-time response
	  is also required.  It also scales down nicely to
	  smaller systems.

	  Select this option if you are unsure.


Christophe

> If you make it full preemption in 6.15 you will likely see similar data 
> as 7.1+.
> Only on 7.0/7.1 there is force switch to lazy/full. Can you give that a 
> try?
>
> If it shows same data, that implies the regression is mainly due to change
> of preemption modes, rather than the static key stuff.
>
> > > Weak memory model would need barriers irrespective of
> > > HAVE_PREEMPT_DYNAMIC_CALL or HAVE_PREEMPT_DYNAMIC_KEY.
> > > Static key too is expected to minimal cost.
> >
> > You're right, I should clarify -- the overhead is not in the static
> > key mechanism itself. The cost comes from __rcu_read_lock() and
> > __rcu_read_unlock() which are called when CONFIG_PREEMPT_RCU=y.
> > These need real memory barriers (lwsync/isync) on ppc64le regardless
> > of whether the dynamic mechanism uses keys or calls.
> >
> > The real question is: why does enabling PREEMPT_RCU cost ~33% on
> > ppc64le but only ~3% on x86_64? The answer is that x86_64's TSO
> > memory model makes the barriers in __rcu_read_lock/__rcu_read_unlock
> > essentially free, while ppc64le's weak ordering requires explicit
> > lwsync/isync instructions, which are expensive when called thousands
> > of times per second in the SELinux AVC hot path.
>
> Plus, it may call schedule in lazy/pull preemption.
>
> > I also have new data from SELinux isolation testing that helps
> > quantify this. On kernel 7.1 (which already has PREEMPT_RCU=y):
> >
> > SELinux mode    kill bogo-ops/sec    vs enforcing
> > ------------    -----------------    ------------
> > Enforcing       69,107               baseline
> > Permissive      70,248               +1.6%
> > Disabled        93,566               +35.4%
> >
> > Permissive ~ enforcing confirms the overhead is not in SELinux policy
> > evaluation. Disabling SELinux removes the rcu_read_lock/unlock call
> > sites in the AVC path and recovers most of the performance -- but
> > still leaves a ~13% gap vs 6.12 (no PREEMPT_RCU), which is the base
> > cost of PREEMPT_RCU in the non-SELinux parts of the kill() path.
>
> This seems strange. How come rcu lock/unlock depends on SELinux policy?
> One should call rcu lock/unlock if they are working with rcu updated 
> fields.
>
> Does the policy change itself protected with rcu lock/unlock?
> I will check it up.
>
> > So the core issue is: on ppc64le, CONFIG_PREEMPT_RCU makes
> > rcu_read_lock/unlock significantly more expensive, and the kill()
> > syscall path hits them very heavily through SELinux AVC lookups.
> >
> > Thanks,
> > Jirka
> >
> > On Mon, Jul 27, 2026 at 6:19 AM Shrikanth Hegde 
> > <sshegde@xxxxxxxxxxxxx> wrote:
> > > +cc paul for any further/RCU insights.
> > >
> > > On 7/27/26 12:03 AM, Jirka Hladky wrote:
> > > >     Hi Shrikanth, Christophe,
> > >
> > > Hi Jirka, thanks for the report.
> > >
> > > >    I'm seeing a significant performance regression on ppc64le after 
> > > > this
> > > >    patch landed in 6.16, caused by CONFIG_PREEMPT_RCU becoming active
> > > >    once HAVE_PREEMPT_DYNAMIC_KEY is selected.
> > >
> > > This is true only if user selected PREEMPT_DYNAMIC option i think.
> > >
> > > config PREEMPT_RCU
> > >           bool
> > >           default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
> > >           select TREE_RCU
> > >
> > >
> > > >    Benchmark: stress-ng kill stressor (tight kill() syscall loop),
> > > >    single thread, POWER10 LPAR (8 vCPUs, 1 core SMT-8).
> > > >
> > > >    Bisected across Fedora ELN kernel builds on ppc64le:
> > > >
> > > >      kernel               CONFIG_PREEMPT_RCU   kill bogo-ops/sec
> > > >    ---  6.15-rc6 (eln148)    no                   103,207
> > > >      6.16     (eln150)          yes                   70,281    (-32%)
> > > >      6.18     (eln154)          yes                   72,552    (-30%)
> > >
> > > Does your preemption mode remain the same in two cases?
> > >
> > > >    For comparison, x86_64 (AMD EPYC 9355P) with the same config change
> > > >    shows only a 2.8% regression:
> > > >
> > > >      6.12 x86_64          37,436
> > > >      7.2  x86_64           36,392    (-2.8%)
> > > >
> > > >    perf report shows the overhead comes from rcu_read_lock/unlock in 
> > > > the
> > > >    SELinux AVC path (check_kill_permission -> security_task_kill ->
> > > >    selinux_task_kill -> avc_has_perm -> avc_lookup):
> > > >
> > > >      Function               6.15 (no PREEMPT_RCU)   6.16 (PREEMPT_RCU)
> > > >    ---  avc_lookup                          15.23%              24.79%
> > > >      __rcu_read_lock                      ~0%                 4.52%
> > > >      __rcu_read_unlock                    ~0%                 4.17%
> > > >      selinux_task_kill                    6.23%               7.35%
> > > >      audit_signal_info*                   0.94%               3.59%
> > > >
> > > >    On x86_64, rcu_read_lock/unlock are cheap thanks to static calls
> > > >    (HAVE_PREEMPT_DYNAMIC_CALL). On ppc64le with the KEY-based
> > > >    implementation, the weak memory model requires real barriers
> > > >    (lwsync/isync) making each RCU read-side critical section
> > > >    significantly more expensive.
> > >
> > > Weak memory model would need barriers irrespective of 
> > > HAVE_PREEMPT_DYNAMIC_CALL
> > > or HAVE_PREEMPT_DYNAMIC_KEY. That's my assumption. I will look
> > > more into it. Also i don't know much about PREEMPT_RCU. So might take 
> > > a while.
> > >
> > > >    This aligns with Christophe's earlier review comment that
> > > >    HAVE_PREEMPT_DYNAMIC_CALL should be more performant. Would
> > > >    implementing static calls for ppc64 be feasible to close this gap?
> > >
> > > Static key too is expected to minimal cost. There maybe more into this.
> > >
> > > >    Test details:
> > > >    - Machine: IBM POWER10 (pvr 0080 0200), pHyp virtualization
> > > >    - stress-ng 0.21.03, gcc 14.3.1, glibc 2.39
> > > >    - Tuned profile: virtual-guest
> > > >    - SELinux: enforcing (permissive recovers only ~7%)
> > > >
> > > >    Happy to run additional tests if needed.