From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7F63D3876C0; Thu, 2 Apr 2026 20:10:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775160622; cv=none; b=udna+ll4X95AAuzjdB0zoeNM+f+uy75ZJp8lvi+wNLQFuzhtcVDaZE7huEpvxpmXZErQSy6+Im6DpFNzJF+NyMxgh/ugj5CqPqbTCDBLhJDoyj10Ok0rLQ+Gq4SZR79Hi5MFialIKQbvd1cIxTgxHCWnvD0ulk9/K4047GWnSKI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775160622; c=relaxed/simple; bh=zQDENnjec6ysDhDoURJTTpTFosFMLCXCK7oSTmuuzOY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Q8QWsxxvGbMXL86lRBpxsSmvovcmLQQCnZlNfVNZMOS92d0Vctw6+6oE8+Uv/kBHpA1vU6EHU+eEfYfYd//m+5n+WtT+CgEvja2uMqAhe/LYf9eerqv6wvTyGa/gzEgqyxJ7OGsLMOy8VZuJmBS4PnuPDUjiEFeaNHzwI+TIN5g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=eqvf+Ekn; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="eqvf+Ekn" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 009B9C19424; Thu, 2 Apr 2026 20:10:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775160622; bh=zQDENnjec6ysDhDoURJTTpTFosFMLCXCK7oSTmuuzOY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=eqvf+EkncHFdVMJ8P67S7H14NL9dQkN0RiO/C6szruFia7+aM52OgwTr0MN+P44wa Aj9Ptuf4dcpouOJH7J7Ps71DP0MiY/xLy9lFIYjHqowWEDblvnJu/wJGeqCPIW28/0 b9EHw7N7R/Ipq+/onqh81BweOvW4KddtPb9Xh6T09jOWdz70KKbrzmDDDu3WfDUSzv cOZ8XlT0zhHX3CTLE4oUQs+zx23dHj2xAxPZjrXkziKQcgjlvvmjDY10k7yXzEmrCH bgAXAjg5BMBS+JkkM4zWkZUqLBmQ+sjvkj5+5a6DmxAeNxjjb92whD0ZDg9+FsUHaj MJRr1YaScqfRg== Date: Thu, 2 Apr 2026 10:10:20 -1000 From: Tejun Heo To: Andrea Righi Cc: David Vernet , Changwoo Min , Daniel Hodges , Patrick Somaru , sched-ext@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] sched_ext: Fix stale direct dispatch state in ddsp_dsq_id Message-ID: References: <20260402085743.1410070-1-arighi@nvidia.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: <20260402085743.1410070-1-arighi@nvidia.com> Hello, Andrea. On Thu, Apr 02, 2026 at 10:57:43AM +0200, Andrea Righi wrote: > @p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a > spurious warning in mark_direct_dispatch() when the next wakeup's > ops.select_cpu() calls scx_bpf_dsq_insert(), such as: > > WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140 > > The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(), > which is not reached in all paths that consume or cancel a direct dispatch > verdict. Instead, clear it at the right places: > > - direct_dispatch(): cache the direct dispatch state in local variables > and clear it before dispatch_enqueue() on the synchronous path. For > the deferred path, the direct dispatch state must remain set until > process_ddsp_deferred_locals() consumes them. > > - process_ddsp_deferred_locals(): cache the dispatch state in local > variables and clear it before calling dispatch_to_local_dsq(), which > may migrate the task to another rq. > > - do_enqueue_task(): clear the dispatch state on the enqueue path > (local/global/bypass fallbacks), where the direct dispatch verdict is > ignored. > > - dequeue_task_scx(): clear the dispatch state after dispatch_dequeue() > to handle both the deferred dispatch cancellation and the holding_cpu > race, covering all cases where a pending direct dispatch is > cancelled. > > - scx_disable_task(): clear the direct dispatch state when > transitioning a task out of the current scheduler. Waking tasks may > have had the direct dispatch state set by the outgoing scheduler's > ops.select_cpu() and then been queued on a wake_list via > ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such > tasks are not on the runqueue and are not iterated by scx_bypass(), > so their direct dispatch state won't be cleared. Without this clear, > when the new scheduler calls scx_enable_task() for these tasks, any > subsequent ops.select_cpu() call that tries to direct dispatch the > task will trigger the WARN_ON_ONCE() in mark_direct_dispatch(). Can you add an abbreviated version of the above as functio comment on clear_direct_dispatch()? > static void direct_dispatch(struct scx_sched *sch, struct task_struct *p, > u64 enq_flags) > { ... > @@ -1303,6 +1301,12 @@ static void direct_dispatch(struct scx_sched *sch, struct task_struct *p, > if (dsq->id == SCX_DSQ_LOCAL && dsq != &rq->scx.local_dsq) { > unsigned long opss; > > + /* > + * Update the direct dispatch state and keep it until > + * process_ddsp_deferred_locals() consumes it. > + */ > + p->scx.ddsp_enq_flags = ddsp_enq_flags; I know I suggested it but this looks kinda odd. How about we keep the original p->scx.ddsp_enq_flags |= enq_flags above and then do ... Cache enq_flags here? > + clear_direct_dispatch(p); > + dispatch_enqueue(sch, dsq, p, ddsp_enq_flags | SCX_ENQ_CLEAR_OPSS); > } > > static bool scx_rq_online(struct rq *rq) ... > @@ -3147,6 +3155,8 @@ static bool task_dead_and_done(struct task_struct *p) > > lockdep_assert_rq_held(rq); > > + clear_direct_dispatch(p); This is task_dead_and_done(), not scx_disable_task(). Is this intended? Thanks. -- tejun