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
