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Wed, 16 Jul 2025 18:21:47 +0000 (GMT) Message-ID: Date: Wed, 16 Jul 2025 23:51:46 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting To: Peter Zijlstra Cc: Sebastian Andrzej Siewior , =?UTF-8?Q?Andr=C3=A9_Almeida?= , Darren Hart , Davidlohr Bueso , Ingo Molnar , Juri Lelli , Thomas Gleixner , linux-kernel@vger.kernel.org, Valentin Schneider , Waiman Long References: <20250710110011.384614-1-bigeasy@linutronix.de> <20250715163134.pM1J2XO9@linutronix.de> <88edcfdf-2253-4563-a895-6e8bb1625800@linux.ibm.com> <20250716142946.GD905792@noisy.programming.kicks-ass.net> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20250716142946.GD905792@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: T2AwZ0QFCjinDQWgdCRkYAykNc2DfPZj X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNzE2MDE2NSBTYWx0ZWRfXx238qieBafcT jLSqfGjaMC7RCZUVmZ6wgcZQM3dfGe+U9PDmtSqwV2O1JLPbQsYQjR+xG4vGX+bL3eA9vMGDfqy aSZhAsfRJoRJerUgYkmoDZ+LXbVoHacwOHATheGBaaJYtRdrlIWmG8AAtF2/5cmqPcu0/qOVvdP 2u9yCEhVK1YtChqw982yOv5XO+mECWAl9nrWfWM6ZDYEry63GJoYfSZp4QVhQc4NJ9XYlyB178J mXX0pdRSomh+HAROg0lUYod1/6lTEDgmZbmRAAraAocgbxByoy/dY2RSdm9oR099mT9TZDrGFxb Ucv39zcboUnFVDPDK7GBHP935ESm6cvuvowFPQ8Byujv+LMB2lwzvqtUSAzI8bCQDimdx9Lh9Ln nHwLCEfq0BHMWMDZV3WEKf9EvqGQymeHKcZauyVAHrV5UaMknD+DhknUCODnaH2GJLkzpwoa X-Proofpoint-ORIG-GUID: T2AwZ0QFCjinDQWgdCRkYAykNc2DfPZj X-Authority-Analysis: v=2.4 cv=dNSmmPZb c=1 sm=1 tr=0 ts=6877edc0 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=Wb1JkmetP80A:10 a=hLBtojj1w5sSJmacFvYA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-07-16_03,2025-07-16_02,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 mlxlogscore=999 bulkscore=0 lowpriorityscore=0 suspectscore=0 spamscore=0 clxscore=1015 phishscore=0 malwarescore=0 mlxscore=0 adultscore=0 impostorscore=0 classifier=spam authscore=0 authtc=n/a authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2505280000 definitions=main-2507160165 On 7/16/25 19:59, Peter Zijlstra wrote: > On Tue, Jul 15, 2025 at 10:34:24PM +0530, Shrikanth Hegde wrote: > >> I did try again by going to baseline, removed BROKEN and ran below. Which gives us immutable numbers. >> ./perf bench futex hash -Ib512 >> Averaged 1536035 operations/sec (+- 0.11%), total secs = 10 >> Futex hashing: 512 hash buckets (immutable) >> >> So, with -b 512 option, it is around 8-10% less compared to immutable. > > Urgh, can you run perf on that and tell me if this is due to > this_cpu_{inc,dec}() doing local_irq_disable() or the smp_load_acquire() > doing LWSYNC ? It seems like due to rcu and irq enable. Both perf records are collected with -b512. base_futex_immutable_b512 - perf record collected with baseline + remove BROKEN + ./perf bench futex hash -Ib512 per_cpu_futex_hash_b_512 - baseline + series + ./perf bench futex hash -b512 perf diff base_futex_immutable_b512 per_cpu_futex_hash_b_512 # Event 'cycles' # # Baseline Delta Abs Shared Object Symbol # ........ ......... .......................... .................................................... # 21.62% -2.26% [kernel.vmlinux] [k] futex_get_value_locked 0.16% +2.01% [kernel.vmlinux] [k] __rcu_read_unlock 1.35% +1.63% [kernel.vmlinux] [k] arch_local_irq_restore.part.0 +1.48% [kernel.vmlinux] [k] futex_private_hash_put +1.16% [kernel.vmlinux] [k] futex_ref_get 10.41% -0.78% [kernel.vmlinux] [k] system_call_vectored_common 1.24% +0.72% perf [.] workerfn 5.32% -0.66% [kernel.vmlinux] [k] futex_q_lock 2.48% -0.43% [kernel.vmlinux] [k] futex_wait 2.47% -0.40% [kernel.vmlinux] [k] _raw_spin_lock 2.98% -0.35% [kernel.vmlinux] [k] futex_q_unlock 2.42% -0.34% [kernel.vmlinux] [k] __futex_wait 5.47% -0.32% libc.so.6 [.] syscall 4.03% -0.32% [kernel.vmlinux] [k] memcpy_power7 0.16% +0.22% [kernel.vmlinux] [k] arch_local_irq_restore 5.93% -0.18% [kernel.vmlinux] [k] futex_hash 1.72% -0.17% [kernel.vmlinux] [k] sys_futex > > Anyway, I think we can improve both. Does the below help? > > > --- > diff --git a/kernel/futex/core.c b/kernel/futex/core.c > index d9bb5567af0c..8c41d050bd1f 100644 > --- a/kernel/futex/core.c > +++ b/kernel/futex/core.c > @@ -1680,10 +1680,10 @@ static bool futex_ref_get(struct futex_private_hash *fph) > { > struct mm_struct *mm = fph->mm; > > - guard(rcu)(); > + guard(preempt)(); > > - if (smp_load_acquire(&fph->state) == FR_PERCPU) { > - this_cpu_inc(*mm->futex_ref); > + if (READ_ONCE(fph->state) == FR_PERCPU) { > + __this_cpu_inc(*mm->futex_ref); > return true; > } > > @@ -1694,10 +1694,10 @@ static bool futex_ref_put(struct futex_private_hash *fph) > { > struct mm_struct *mm = fph->mm; > > - guard(rcu)(); > + guard(preempt)(); > > - if (smp_load_acquire(&fph->state) == FR_PERCPU) { > - this_cpu_dec(*mm->futex_ref); > + if (READ_ONCE(fph->state) == FR_PERCPU) { > + __this_cpu_dec(*mm->futex_ref); > return false; > } > Yes. It helps. It improves "-b 512" numbers by at-least 5%. baseline + series: Averaged 1412543 operations/sec (+- 0.14%), total secs = 10 Futex hashing: 512 hash buckets baseline + series+ above_patch: Averaged 1482733 operations/sec (+- 0.26%), total secs = 10 <<< 5% improvement Futex hashing: 512 hash buckets Now we are closer baseline/immutable by 4-5%. baseline: commit 8784fb5fa2e0042fe3b1632d4876e1037b695f56 (HEAD) ./perf bench futex hash Averaged 1559643 operations/sec (+- 0.09%), total secs = 10 Futex hashing: global hash