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client-ip=165.204.84.17; helo=satlexmb07.amd.com; pr=C Received: from satlexmb07.amd.com (165.204.84.17) by CY4PEPF0000EE3C.mail.protection.outlook.com (10.167.242.13) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9499.1 via Frontend Transport; Mon, 5 Jan 2026 06:41:50 +0000 Received: from satlexmb08.amd.com (10.181.42.217) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.17; Mon, 5 Jan 2026 00:41:49 -0600 Received: from [10.136.46.19] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.17 via Frontend Transport; Mon, 5 Jan 2026 00:41:47 -0600 Message-ID: Date: Mon, 5 Jan 2026 12:11:41 +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 2/3] sched/fair: Change likelyhood of nohz.nr_cpus and do stats update if its due To: Shrikanth Hegde CC: , , , , , , , , , , References: <20260102124744.360872-1-sshegde@linux.ibm.com> <20260102124744.360872-3-sshegde@linux.ibm.com> <880e3610-dbdb-42a8-9ddb-ab2e7d3cdc1f@linux.ibm.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <880e3610-dbdb-42a8-9ddb-ab2e7d3cdc1f@linux.ibm.com> Content-Type: text/plain; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 05 Jan 2026 06:41:50.6233 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 62663cae-fe03-40ca-6571-08de4c2581b9 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: CY4PEPF0000EE3C.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB9491 Hello Shrikanth, On 1/5/2026 10:37 AM, Shrikanth Hegde wrote: >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -12456,10 +12456,10 @@ static void nohz_balancer_kick(struct rq *rq) >>>         /* >>>        * None are in tickless mode and hence no need for NOHZ idle load >>> -     * balancing: >>> +     * balancing, do stats update if its due >>>        */ >>> -    if (likely(!atomic_read(&nohz.nr_cpus))) >>> -        return; >>> +    if (unlikely(!atomic_read(&nohz.nr_cpus))) >>> +        goto out; > > Did something got edited here? Welp! Stray edit. My bad. Reverted back to original diff. > >> Since we are sure that "nohz.nr_cpus" is 0, there is a good chance >> find_new_ilb() in kick_ilb() will not find any CPU to run balance on, so >> why not just retain that return? >> >> The "flags" can only be set to (NOHZ_NEXT_KICK | NOHZ_STATS_KICK) on >> this path and kick_ilb() will simply return early without updating >> "nohz.next_balance" when it doesn't see NOHZ_BALANCE_KICK and fails to >> find any CPU. Might as well keep the early return. >> > > The only reason why flags would be set is, if nohz.has_blocked_load > is set and time is after next_blocked. In that case, doing a stats > based balance will make nohz.has_blocked_load=0 and subsequent invocations > flags =0 and no load balance will happen if nr_cpus stays 0. > > However, if we just, has_blocked_load might remains stale value. > > Isn't that the case? So cumulatively, including Patch 3, we do: flags = 0; if (READ_ONCE(nohz.has_blocked_load) && ...) flags = NOHZ_STATS_KICK; if (time_before(now, nohz.next_balance)) goto out; /* Checks nohz.idle_cpus_mask in find_new_ilb() ... (1) */ if (unlikely(cpumask_empty(nohz.idle_cpus_mask))) goto out; /* Still goes to kick_ilb() ... (2) */ ... out: if (READ_ONCE(nohz.needs_update)) flags |= NOHZ_NEXT_KICK; /* assume either NOHZ_STATS_KICK or NOHZ_NEXT_KICK is set */ kick_ilb() { if (flags & NOHZ_BALANCE_KICK) /* Not possible */ ... ilb_cpu = find_new_ilb(); /* Find CPU in nohz.idle_cpus_mask */ If we arrive here from (2), we know "nohz.idle_cpus_mask" was empty a while back and we've not updated any global "nohz" state. If we don't find an ilb_cpu, we just do: if (ilb_cpu < 0) return; So why not simply return from (2)? -- Thanks and Regards, Prateek