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Sashiko AI review found 1 potential issue(s) to consider: > - [High] sched_ext: balance callbacks queued before core-sched pick is final > -- > > commit 859b8e499322d8a16b0b359431e360cdd5ae7494 > Author: Andrea Righi > > sched_ext: Don't leave balance callbacks queued across a pick retry > > This commit moves the queueing of rq->scx.deferred_bal_cb and > rq->scx.kick_sync_bal_cb to the exit path of do_pick_task_scx() so that > balance callbacks are only queued when do_pick_task_scx() returns a final > task rather than RETRY_TASK or a NULL dl_server pick. > > > diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c > > index 0403c46342813..e19961164cb01 100644 > > --- a/kernel/sched/ext/ext.c > > +++ b/kernel/sched/ext/ext.c > > [ ... ] > > > @@ -3342,16 +3331,8 @@ static enum scx_dsp_verdict dispatch_core_pick(struct rq *rq, struct rq_flags *r > > > > verdict = dispatch_one(rq, prev); > > > > - if (cpu_of(rq) == smp_processor_id()) { > > - maybe_queue_balance_callback(rq); > > - > > - /* see dispatch_pick() */ > > - if (unlikely(rq->scx.kick_sync_pending)) { > > - rq->scx.kick_sync_pending = false; > > - queue_balance_callback(rq, &rq->scx.kick_sync_bal_cb, > > - kick_sync_wait_bal_cb); > > - } > > - } else if (unlikely(rq->scx.flags & SCX_RQ_BAL_CB_PENDING)) { > > + if (cpu_of(rq) != smp_processor_id() && > > + unlikely(rq->scx.flags & SCX_RQ_BAL_CB_PENDING)) { > > /* > > * Balance callbacks must run in the context that queued them, > > * so they can't be queued on another CPU's rq. Run the deferred > > @@ -3394,8 +3375,10 @@ do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx) > > else > > verdict = dispatch_pick(rq, rf, prev); > > > > - if (verdict == SCX_DSP_RETRY) > > - return RETRY_TASK; > > + if (verdict == SCX_DSP_RETRY) { > > + p = RETRY_TASK; > > + goto out; > > + } > > [ ... ] > > > @@ -3437,6 +3422,38 @@ do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx) > > } > > } > > > > +out: > > + if (cpu_of(rq) != smp_processor_id()) > > + return p; > > + > > + /* > > + * Balance callbacks must be consumed before the rq lock is released, so > > + * queue them only when the pick is final. On RETRY_TASK, or after a > > + * NULL pick from the dl_server, the core continues picking and can > > + * release the rq lock (e.g., prev_balance() or another dispatch). In > > + * that case, run the deferred work from irq_work, which doesn't depend > > + * on the current rq lock section, and leave the kick sync wait pending > > + * for the next pick. > > + */ > > + if (p != RETRY_TASK && (p || !force_scx)) { > > + maybe_queue_balance_callback(rq); > > + > > + /* > > + * Defer to a balance callback which can drop rq lock and enable > > + * IRQs. Waiting directly in the pick path would deadlock against > > + * CPUs sending us IPIs (e.g., TLB flushes) while we wait for > > + * them. > > + */ > > + if (unlikely(rq->scx.kick_sync_pending)) { > > + rq->scx.kick_sync_pending = false; > > + queue_balance_callback(rq, &rq->scx.kick_sync_bal_cb, > > + kick_sync_wait_bal_cb); > > + } > > + } else if (rq->scx.flags & SCX_RQ_BAL_CB_PENDING) { > > + rq->scx.flags &= ~SCX_RQ_BAL_CB_PENDING; > > + schedule_deferred(rq); > > + } > > + > > return p; > > } > > [Severity: High] > Under CONFIG_SCHED_CORE, can a non-RETRY_TASK return from > do_pick_task_scx() on the local rq still be followed by further picks that > release the shared core lock before the core-wide pick is final? > > In kernel/sched/core.c:pick_next_task(), a single shared core lock is held > across all SMT siblings, and pick_task() can be called multiple times after > the local rq pick succeeds: > > 1. In the !need_sync fastpath, pick_task(rq, rf) runs on the local rq: > > kernel/sched/core.c:pick_next_task() { > if (!need_sync) { > next = pick_task(rq, rf); > ... > if (!next->core_cookie) { > ... > goto out_set_next; > } > } > } > > If do_pick_task_scx() returns a cookied SCX task (next->core_cookie != 0), > the out: block sees p != RETRY_TASK && (p || !force_scx) and queues > rq->scx.deferred_bal_cb and/or rq->scx.kick_sync_bal_cb on > rq->balance_callback. Because next->core_cookie is non-zero, > pick_next_task() then falls through into the for_each_cpu_wrap(i, smt_mask, > cpu) loop to re-pick across all siblings. > > 2. In the for_each_cpu_wrap(i, smt_mask, cpu) loop in pick_next_task(), > iteration starts at i = cpu (where cpu_of(rq_i) == smp_processor_id()): > > kernel/sched/core.c:pick_next_task() { > for_each_cpu_wrap(i, smt_mask, cpu) { > rq_i = cpu_rq(i); > ... > p = pick_task(rq_i, rf); > if (unlikely(p == RETRY_TASK)) { > ... > goto restart; > } > } > } > > Once do_pick_task_scx() on the local rq succeeds and queues the balance > callbacks on rq->balance_callback, the loop proceeds to call > pick_task(rq_i, rf) on the remaining SMT siblings (i != cpu). When > dispatch_core_pick(rq_i, rf, prev) runs for a sibling rq_i, > dispatch_one(rq_i, prev) or run_deferred(rq_i) can drop the shared core > lock and return SCX_DSP_RETRY, causing pick_next_task() to jump to restart > with rq->balance_callback already queued on the local rq. Any CPU acquiring > the core lock during that window will trigger the warning in > assert_balance_callbacks_empty() via rq_pin_lock() or flush the callback > early. I'm planning to fix this with a follow-up fix, it was also pointed out by Tejun in his review of v1. > > 3. Also in dispatch_core_pick(), if an SMT sibling already has an in-flight > dispatch on the local rq: > > kernel/sched/ext/ext.c:dispatch_core_pick() { > /* another dispatch is in flight on @rq, let that handle it */ > if (rq->scx.flags & SCX_RQ_IN_DISPATCH) > return SCX_DSP_NONE; > } > > Previously, returning SCX_DSP_NONE here bypassed callback queueing in > dispatch_core_pick(). With the callback handling moved to the out: label in > do_pick_task_scx(), won't this early return now fall through to out: and > consume SCX_RQ_BAL_CB_PENDING and rq->scx.kick_sync_pending while the > sibling's dispatch_one() on rq is still in flight? Hm... yeah this looks legit for SCX_RQ_BAL_CB_PENDING, I'll fix this in v3. -Andrea