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Tue, 14 Oct 2025 09:51:01 +0000 (GMT) Message-ID: Date: Tue, 14 Oct 2025 15:21:01 +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: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg when balance is not due To: Peter Zijlstra Cc: Tim Chen , Ingo Molnar , Chen Yu , Doug Nelson , Mohini Narkhede , linux-kernel@vger.kernel.org, Vincent Guittot , K Prateek Nayak References: <20251013142638.GM3245006@noisy.programming.kicks-ass.net> <20251014092436.GK4067720@noisy.programming.kicks-ass.net> <20251014094259.GR3245006@noisy.programming.kicks-ass.net> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: <20251014094259.GR3245006@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-ORIG-GUID: a7HF92he2dabb12W6v0m6oaHv2P7faTI X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMDExMDAxMSBTYWx0ZWRfXywFECmkxUqK3 mkN2YqTZlWgZbUPOqsNTBrmgV5WbOQutfdtaJ6gUH9RExfQh78mzVxK/zhsgAzx/xRZp71szf8o hYpMJ8VyIOQBqkcfmXlt35OApQE0I/qNV1Nn1lXpgxVz+r4fgayg4Oiz633t7mhQGBd425f7JI5 GrTgs1Q3u5wvF+9aU1VGkgOnhOgWp/TCYYJ0OAj0thhtwbLxzh5qans+t5ILoXilT8uFusXPrkI UAOhUGnErRu4UCG1BOSgSrWsyotDzHGkKHtU/kivU9RlbhnwVfzyPld0LAlLQXdZi5quOS0TaJg XFewc9Fk9HgizjXdHsQbjl8aXWkBsRqXkTVlJMYnS7+AegP9Lce3a6MP171vbZSUKJOgOzn2W8O DVqtNFy257dbicg7ZavbSdv5RlyUwA== X-Proofpoint-GUID: a7HF92he2dabb12W6v0m6oaHv2P7faTI X-Authority-Analysis: v=2.4 cv=QZ5rf8bv c=1 sm=1 tr=0 ts=68ee2245 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=x6icFKpwvdMA:10 a=VkNPw1HP01LnGYTKEx00:22 a=fN0QsRKp1KExyfdZop8A:9 a=QEXdDO2ut3YA:10 a=cPQSjfK2_nFv0Q5t_7PE:22 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-10-14_02,2025-10-13_01,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 priorityscore=1501 phishscore=0 bulkscore=0 suspectscore=0 spamscore=0 malwarescore=0 impostorscore=0 clxscore=1015 adultscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2510020000 definitions=main-2510110011 On 10/14/25 3:12 PM, Peter Zijlstra wrote: > On Tue, Oct 14, 2025 at 03:03:41PM +0530, Shrikanth Hegde wrote: > >>> @@ -11758,6 +11775,12 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq, >>> goto out_balanced; >>> } >>> + if (idle != CPU_NEWLY_IDLE && (sd->flags & SD_SERIALIZE)) { >>> + if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1)) >>> + goto out_balanced; >> >> Maybe goto out instead of out_balanced ? > > That would be inconsistent with the !should_we_balance() goto > out_balanced right above this, no? Yes. But whats the reason for saying out_balanced for !should_we_balance? Load balance wasn't even attempted there right? Could this be updating it wrongly? At-least comments around out_all_pinned doesn't make sense if we came here via !swb schedstat_inc(sd->lb_balanced[idle]); sd->nr_balance_failed = 0; > >>> + need_unlock = true; >>> + } >>> + >>> group = sched_balance_find_src_group(&env); >>> if (!group) { >>> schedstat_inc(sd->lb_nobusyg[idle]); >>> @@ -11998,6 +12021,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq, >>> sd->balance_interval < sd->max_interval) >>> sd->balance_interval *= 2; >>> out: >>> + if (need_unlock) >>> + atomic_set_release(&sched_balance_running, 0); >>> + >>> return ld_moved; >>> } >>> @@ -12122,21 +12148,6 @@ static int active_load_balance_cpu_stop(void *data) >>> return 0; >>> } >>> -/* >>> - * This flag serializes load-balancing passes over large domains >>> - * (above the NODE topology level) - only one load-balancing instance >>> - * may run at a time, to reduce overhead on very large systems with >>> - * lots of CPUs and large NUMA distances. >>> - * >>> - * - Note that load-balancing passes triggered while another one >>> - * is executing are skipped and not re-tried. >>> - * >>> - * - Also note that this does not serialize rebalance_domains() >>> - * execution, as non-SD_SERIALIZE domains will still be >>> - * load-balanced in parallel. >>> - */ >>> -static atomic_t sched_balance_running = ATOMIC_INIT(0); >>> - >>> /* >>> * Scale the max sched_balance_rq interval with the number of CPUs in the system. >>> * This trades load-balance latency on larger machines for less cross talk. >>> @@ -12192,7 +12203,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle) >>> /* Earliest time when we have to do rebalance again */ >>> unsigned long next_balance = jiffies + 60*HZ; >>> int update_next_balance = 0; >>> - int need_serialize, need_decay = 0; >>> + int need_decay = 0; >>> u64 max_cost = 0; >>> rcu_read_lock(); >>> @@ -12216,13 +12227,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle) >>> } >>> interval = get_sd_balance_interval(sd, busy); >>> - >>> - need_serialize = sd->flags & SD_SERIALIZE; >>> - if (need_serialize) { >>> - if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1)) >>> - goto out; >>> - } >>> - >>> if (time_after_eq(jiffies, sd->last_balance + interval)) { >>> if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) { >>> /* >>> @@ -12236,9 +12240,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle) >>> sd->last_balance = jiffies; >>> interval = get_sd_balance_interval(sd, busy); >>> } >>> - if (need_serialize) >>> - atomic_set_release(&sched_balance_running, 0); >>> -out: >>> + >>> if (time_after(next_balance, sd->last_balance + interval)) { >>> next_balance = sd->last_balance + interval; >>> update_next_balance = 1; >>