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
