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Tsirkin" To: Tobias Huschle Cc: Jason Wang , Abel Wu , Peter Zijlstra , Linux Kernel , kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org Subject: Re: Re: Re: EEVDF/vhost regression (bisected to 86bfbb7ce4f6 sched/fair: Add lag based placement) Message-ID: <20240201070437-mutt-send-email-mst@kernel.org> References: <42870.123121305373200110@us-mta-641.us.mimecast.lan> <20231213061719-mutt-send-email-mst@kernel.org> <25485.123121307454100283@us-mta-18.us.mimecast.lan> <20231213094854-mutt-send-email-mst@kernel.org> <20231214021328-mutt-send-email-mst@kernel.org> <92916.124010808133201076@us-mta-622.us.mimecast.lan> <20240121134311-mutt-send-email-mst@kernel.org> <07974.124020102385100135@us-mta-501.us.mimecast.lan> <20240201030341-mutt-send-email-mst@kernel.org> <89460.124020106474400877@us-mta-475.us.mimecast.lan> Precedence: bulk X-Mailing-List: netdev@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: <89460.124020106474400877@us-mta-475.us.mimecast.lan> On Thu, Feb 01, 2024 at 12:47:39PM +0100, Tobias Huschle wrote: > On Thu, Feb 01, 2024 at 03:08:07AM -0500, Michael S. Tsirkin wrote: > > On Thu, Feb 01, 2024 at 08:38:43AM +0100, Tobias Huschle wrote: > > > On Sun, Jan 21, 2024 at 01:44:32PM -0500, Michael S. Tsirkin wrote: > > > > On Mon, Jan 08, 2024 at 02:13:25PM +0100, Tobias Huschle wrote: > > > > > On Thu, Dec 14, 2023 at 02:14:59AM -0500, Michael S. Tsirkin wrote: > > > > > > -------- Summary -------- > > > > > > In my (non-vhost experience) opinion the way to go would be either > > > replacing the cond_resched with a hard schedule or setting the > > > need_resched flag within vhost if the a data transfer was successfully > > > initiated. It will be necessary to check if this causes problems with > > > other workloads/benchmarks. > > > > Yes but conceptually I am still in the dark on whether the fact that > > periodically invoking cond_resched is no longer sufficient to be nice to > > others is a bug, or intentional. So you feel it is intentional? > > I would assume that cond_resched is still a valid concept. > But, in this particular scenario we have the following problem: > > So far (with CFS) we had: > 1. vhost initiates data transfer > 2. kworker is woken up > 3. CFS gives priority to woken up task and schedules it > 4. kworker runs > > Now (with EEVDF) we have: > 0. In some cases, kworker has accumulated negative lag > 1. vhost initiates data transfer > 2. kworker is woken up > -3a. EEVDF does not schedule kworker if it has negative lag > -4a. vhost continues running, kworker on same CPU starves > -- > -3b. EEVDF schedules kworker if it has positive or no lag > -4b. kworker runs > > In the 3a/4a case, the kworker is given no chance to set the > necessary flag. The flag can only be set by another CPU now. > The schedule of the kworker was not caused by cond_resched, but > rather by the wakeup path of the scheduler. > > cond_resched works successfully once the load balancer (I suppose) > decides to migrate the vhost off to another CPU. In that case, the > load balancer on another CPU sets that flag and we are good. > That then eventually allows the scheduler to pick kworker, but very > late. I don't really understand what is special about vhost though. Wouldn't it apply to any kernel code? > > I propose a two patch series then: > > > > patch 1: in this text in Documentation/kernel-hacking/hacking.rst > > > > If you're doing longer computations: first think userspace. If you > > **really** want to do it in kernel you should regularly check if you need > > to give up the CPU (remember there is cooperative multitasking per CPU). > > Idiom:: > > > > cond_resched(); /* Will sleep */ > > > > > > replace cond_resched -> schedule > > > > > > Since apparently cond_resched is no longer sufficient to > > make the scheduler check whether you need to give up the CPU. > > > > patch 2: make this change for vhost. > > > > WDYT? > > For patch 1, I would like to see some feedback from Peter (or someone else > from the scheduler maintainers). I am guessing once you post it you will see feedback. > For patch 2, I would prefer to do some more testing first if this might have > an negative effect on other benchmarks. > > I also stumbled upon something in the scheduler code that I want to verify. > Maybe a cgroup thing, will check that out again. > > I'll do some more testing with the cond_resched->schedule fix, check the > cgroup thing and wait for Peter then. > Will get back if any of the above yields some results. > > > > > -- > > MST > > > >