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[35.195.168.105]) by smtp.gmail.com with ESMTPSA id l9sm5397611wme.21.2021.06.11.02.07.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Jun 2021 02:07:42 -0700 (PDT) Date: Fri, 11 Jun 2021 09:07:40 +0000 From: Quentin Perret To: Peter Zijlstra Cc: mingo@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, qais.yousef@arm.com, rickyiu@google.com, wvw@google.com, patrick.bellasi@matbug.net, xuewen.yan94@gmail.com, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS Message-ID: References: <20210610151306.1789549-1-qperret@google.com> <20210610151306.1789549-3-qperret@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Friday 11 Jun 2021 at 08:59:25 (+0000), Quentin Perret wrote: > On Thursday 10 Jun 2021 at 21:15:45 (+0200), Peter Zijlstra wrote: > > On Thu, Jun 10, 2021 at 03:13:05PM +0000, Quentin Perret wrote: > > > SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that > > > the call must not touch scheduling parameters (nice or priority). This > > > is particularly handy for uclamp when used in conjunction with > > > SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only > > > impacts uclamp values. > > > > > > However, sched_setattr always checks whether the priorities and nice > > > values passed in sched_attr are valid first, even if those never get > > > used down the line. This is useless at best since userspace can > > > trivially bypass this check to set the uclamp values by specifying low > > > priorities. However, it is cumbersome to do so as there is no single > > > expression of this that skips both RT and CFS checks at once. As such, > > > userspace needs to query the task policy first with e.g. sched_getattr > > > and then set sched_attr.sched_priority accordingly. This is racy and > > > slower than a single call. > > > > > > As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS > > > is specified, simply inherit them in this case to match the policy > > > inheritance of SCHED_FLAG_KEEP_POLICY. > > > > > > Reported-by: Wei Wang > > > Signed-off-by: Quentin Perret > > > --- > > > kernel/sched/core.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > > index 3b213402798e..1d4aedbbcf96 100644 > > > --- a/kernel/sched/core.c > > > +++ b/kernel/sched/core.c > > > @@ -6585,6 +6585,10 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr, > > > rcu_read_unlock(); > > > > > > if (likely(p)) { > > > + if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS) { > > > + attr.sched_priority = p->rt_priority; > > > + attr.sched_nice = task_nice(p); > > > + } > > > retval = sched_setattr(p, &attr); > > > put_task_struct(p); > > > } > > > > I don't like this much... afaict the KEEP_PARAMS clause in > > __setscheduler() also covers the DL params, and you 'forgot' to copy > > those. > > > > Can't we short circuit the validation logic? > > I think we can but I didn't like the look of it, because we end up > sprinkling checks all over the place. KEEP_PARAMS doesn't imply > KEEP_POLICY IIUC, and the policy and params checks are all mixed up. > > But maybe that wants fixing too? I guess it could make sense to switch > policies without touching the params in some cases (e.g switching > between FIFO and RR, or BATCH and NORMAL), but I'm not sure what that > would mean for cross-sched_class transitions. Aha, policy transitions are actually blocked in __setscheduler if KEEP_PARAMS is set, so KEEP_PARAMS does imply KEEP_POLICY. So skipping the checks might not be too bad, I'll have a go at it.