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Fri, 25 Sep 2026 08:07:54 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([176.206.14.6]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4887a744fb5sm7133085f8f.33.2026.09.25.08.07.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Sep 2026 08:07:53 -0700 (PDT) Date: Fri, 25 Sep 2026 17:07:51 +0200 From: Juri Lelli To: "Ionut Nechita (Wind River)" Cc: mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, arighi@nvidia.com, frederic@kernel.org, linux-kernel@vger.kernel.org, dhaufe@simplextrading.com, create0818@163.com, frn1furkan10@gmail.com Subject: Re: [PATCH v2] sched/deadline: Make dl-server nohz full aware Message-ID: References: <20260513-upstream-fix-dlserver-nohzfull-b4-v2-1-d3e9cbe5c845@redhat.com> <20260924170402.521623-1-ionut.nechita@windriver.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260924170402.521623-1-ionut.nechita@windriver.com> Hi Ionut, On 24/09/26 20:02, Ionut Nechita (Wind River) wrote: > Hi Juri, > > I have been chasing timer noise on isolated nohz_full cores for an RT > product and ended up on this patch. It does restore the CFS bandwidth > guarantee, but on the isolated core itself it trades the dl-server's > timers for a full CONFIG_HZ tick, which for us is the more expensive of > the two. > > Below are measurements from two machines, a variant that keeps the core > tickless, and a hazard in the dl_servers_stop_all() call sites that an > equivalent change of mine ran into. The variant is not a replacement for > your patch as it stands, since it does not address the housekeeping > wakeups you are fixing, so this is more a "can we get both?" than a > counter-proposal. Thanks for the detailed analysis! I believe my v2 was never picked up, so I'm happy for you to send a v3 modified with your approach. Keeping the tick stopped during the server's throttled window sounds reasonable to me. Having a proper single patch will make it easier to evaluate. Please do include the ext_server handling as well so we have the complete picture. ... > A separate observation > ====================== > > Independently of either patch, the CFS wakeup pattern changed between > v6.12 and v6.18. With the same workload, v6.12 serves the periodic CFS > task every ~50ms (199 wakeups in 10s), while both v6.18 variants on that > machine serve it once per server period (10 wakeups in 10s, ~1s worst > case). The bandwidth is the same, but it arrives in one burst per period > instead of being spread out. For an isolated core running a periodic > housekeeping task next to an RT application, that is a user-visible > change. Is the deferred activation expected to behave this way, or is it > worth looking at separately? I think what you see is the deferred server model at work. The server now defers activation until zero-laxity. The dl-server is primarily a safety net to prevent complete CFS starvation under RT, not a latency guarantee, so the bandwidth being correct (5%) is what matters and how it's distributed within the period is a secondary concern. That said, if I understand your example correctly, that periodic housekeeping CFS task you have on the same isolated core of the RT application is relying on dl-server to be able to run? It doesn't seem a safe approach to me, the RT application should better ensure to sleep at times leaving space for CFS housekeeping task to execute w/o activating the dl-server? Best, Juri