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McKenney" References: <20250210184334.567383-1-sshegde@linux.ibm.com> <20250210184334.567383-2-sshegde@linux.ibm.com> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Info: AW1haW4tMjYwNzI3MDAzOSBTYWx0ZWRfX0dyuVLO7WMA6 waj8ATPcBTCocJ4/0195AvdHHf/7CGCoBEgsPOhfd2aquSKMpxQQF8Bf0LocLZ5TzPkalAa/mCn nuhJiZ9hVCr4TCp0VMdKCUhM1Gh3OlE= X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNzI3MDAzOSBTYWx0ZWRfX5Syog1p798Je Wj6X8JEm40AahqWASLaPg2OrIxw4OAhi6upIg4Y9siDCKbK3W9Evn9ej+n2yUAbj8kNla6tizpI 2zXJWcwMRWwKgdZlHxSLsalSzBxE98NSWzEax3znGfjaTAnI6ktMM5HXgQ1dCFd6R9cr9tJ1Ynn tYyww+mNMmgYVmBfec2EiAiWMmoTU/phL8GoCXojNyxA9LIuapFjf4wztLvQ6uuMs50ma5rbBVd IQApIw/AcK1V2QcNG6WLFr2gu+2xJ6bnrXuCJtu4GqkyLsfquI8xnnXud2sF8p7xKDGfCHn4aAh 4cEW+ZivGZcNE8ATwxzuIvjsws7hAOc7cCxuFCl0VVyfzrwzl0nG4lggp2TyRc+wnt2QBz3T7qC ROT8yLGBSWcxbDztmXyTlAdgDdxXXIHOAWmDt4TvNyqwY4ErLmUbWfDy5gnYjQHWimCuJKKeLAK UgzbvQUISBnZFydX+wA== X-Authority-Analysis: v=2.4 cv=b5WCJNGx c=1 sm=1 tr=0 ts=6a66dc18 cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=IkcTkHD0fZMA:10 a=RAioF0-LDSMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=WCvXFVo8NGMObP1iTCQA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: Y7O2bbFrWB8ZbNPYQkJ4CreFfKHts4oh X-Proofpoint-ORIG-GUID: V0sD5WmVoqZRmlE5UDM33GA9tPAK-UOA X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-07-27_01,2026-07-24_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 spamscore=0 adultscore=0 malwarescore=0 impostorscore=0 bulkscore=0 phishscore=0 suspectscore=0 clxscore=1011 lowpriorityscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2607270039 +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. > >