From: Tejun Heo Date: Wed, 7 Aug 2024 22:13:38 +0000 (-1000) Subject: sched_ext: Make scx_rq_online() also test cpu_active() in addition to SCX_RQ_ONLINE X-Git-Tag: v6.12-rc1~111^2~43 X-Git-Url: https://git.kernel.dk/?a=commitdiff_plain;h=991ef53a4832941c8130008ef35c66ec88c7fa0f;p=linux-block.git sched_ext: Make scx_rq_online() also test cpu_active() in addition to SCX_RQ_ONLINE scx_rq_online() currently only tests SCX_RQ_ONLINE. This isn't fully correct - e.g. consume_dispatch_q() uses task_run_on_remote_rq() which tests scx_rq_online() to see whether the current rq can run the task, and, if so, calls consume_remote_task() to migrate the task to @rq. While the test itself was done while locking @rq, @rq can be temporarily unlocked by consume_remote_task() and nothing prevents SCX_RQ_ONLINE from going offline before the migration takes place. To address the issue, add cpu_active() test to scx_rq_online(). There is a synchronize_rcu() between cpu_active() being cleared and the rq going offline, so if an on-going scheduling operation sees cpu_active(), the associated rq is guaranteed to not go offline until the scheduling operation is complete. Signed-off-by: Tejun Heo Fixes: 60c27fb59f6c ("sched_ext: Implement sched_ext_ops.cpu_online/offline()") Acked-by: David Vernet --- diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 9efb54172495..17af9c46d891 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -1818,7 +1818,14 @@ dispatch: static bool scx_rq_online(struct rq *rq) { - return likely(rq->scx.flags & SCX_RQ_ONLINE); + /* + * Test both cpu_active() and %SCX_RQ_ONLINE. %SCX_RQ_ONLINE indicates + * the online state as seen from the BPF scheduler. cpu_active() test + * guarantees that, if this function returns %true, %SCX_RQ_ONLINE will + * stay set until the current scheduling operation is complete even if + * we aren't locking @rq. + */ + return likely((rq->scx.flags & SCX_RQ_ONLINE) && cpu_active(cpu_of(rq))); } static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,