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Date: Fri, 02 Oct 2026 12:33:27 -1000
Message-ID: <86a16fc6ea0a57cea7459ac2ebfdfc1b@kernel.org>
From: Tejun Heo <tj@kernel.org>
To: Kuba Piecuch <jpiecuch@google.com>
Cc: Andrea Righi <arighi@nvidia.com>, Changwoo Min <changwoo@igalia.com>,
	David Vernet <void@manifault.com>, linux-kernel@vger.kernel.org,
	sched-ext@lists.linux.dev
Subject: Re: [PATCH sched_ext/for-7.3-fixes] sched_ext: Generate qseq from a
 per-task counter
In-Reply-To: <20261002205242.3820674-1-jpiecuch@google.com>
References: <20261002205242.3820674-1-jpiecuch@google.com>
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Hello, Kuba.

On Fri, Oct 02, 2026 at 08:52:41PM +0000, Kuba Piecuch wrote:
> +	do {
> +		p->scx.ops_qseq++;
> +		qseq = (unsigned long)p->scx.ops_qseq << SCX_OPSS_QSEQ_SHIFT;
> +	} while (unlikely(!qseq));

Maybe wrap the counter where the QSEQ field wraps instead of looping?

	p->scx.ops_qseq = ((p->scx.ops_qseq + 1) &
			   (SCX_OPSS_QSEQ_MASK >> SCX_OPSS_QSEQ_SHIFT)) ?: 1;
	qseq = (unsigned long)p->scx.ops_qseq << SCX_OPSS_QSEQ_SHIFT;

On 32bit, the shift drops the top two bits of the counter, so the mask is
what keeps the counter from reaching a value that shifts to 0.

Thanks.

-- 
tejun