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Date: Fri, 28 Aug 2026 14:04:08 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: David Stevens <stevensd@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Thomas Gleixner <tglx@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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	"H . Peter Anvin" <hpa@zytor.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dave Chinner <david@fromorbit.com>, Qi Zheng <qi.zheng@linux.dev>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
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	Uladzislau Rezki <urezki@gmail.com>,
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	"Liam R . Howlett" <liam@infradead.org>,
	Vlastimil Babka <vbabka@kernel.org>,
	Mike Rapoport <rppt@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Michal Hocko <mhocko@suse.com>, Kees Cook <kees@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Clark Williams <clrkwllms@kernel.org>, suleiman@google.com,
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mm@kvack.org, linux-rt-devel@lists.linux.dev
Subject: Re: [RFC 06/10] Reclaim memory from blocked kernel stacks
Message-ID: <20260828120408.GN687043@noisy.programming.kicks-ass.net>
References: <20260827232948.2520558-1-stevensd@google.com>
 <20260827232948.2520558-7-stevensd@google.com>
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In-Reply-To: <20260827232948.2520558-7-stevensd@google.com>

On Thu, Aug 27, 2026 at 04:29:44PM -0700, David Stevens wrote:
> @@ -4320,8 +4319,18 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  		 * A similar smp_rmb() lives in __task_needs_rq_lock().
>  		 */
>  		smp_rmb();
> -		if (READ_ONCE(p->on_rq) && ttwu_runnable(p, wake_flags))
> +		if (READ_ONCE(p->on_rq) && ttwu_runnable(p, wake_flags)) {
> +			trace_sched_waking(p);
> +			break;
> +		}
> +
> +		if (!ensure_stack_is_present(p, &need_deferred_repopulate)) {
> +			WRITE_ONCE(p->__state, TASK_STACK_RECLAIM);
> +			do_deferred_repopulate_wake = need_deferred_repopulate;
>  			break;
> +		}
> +
> +		trace_sched_waking(p);

Absolutely not; ensure_stack_is_present() must not call
repopulate_stack() while holding ->pi_lock. Not happening.