linux-kernel - Re: [PATCH] trace/pid_list: optimize pid

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Message-ID: <20251112104211.353de586@gandalf.local.home>
Date: Wed, 12 Nov 2025 10:42:11 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Yongliang Gao <leonylgao@...il.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Yongliang Gao <leonylgao@...cent.com>,
 Huang Cun <cunhuang@...cent.com>, frankjpliu@...cent.com
Subject: Re: [PATCH] trace/pid_list: optimize pid_list->lock contention

On Wed, 12 Nov 2025 13:27:10 +0800
Yongliang Gao <leonylgao@...il.com> wrote:

> Thank you for your detailed response and the proposed RCU-like approach.
> 
> I've looked into using a regular seqlock instead of the current
> implementation, but as you pointed out, the write side is indeed a
> critical path. More importantly, I found that even with seqlock, the
> write_seqlock() function internally uses spin_lock() which on
> PREEMPT_RT gets converted to an mutex. This would cause the same
> issues we're trying to avoid - potential sleep in atomic contexts.

I believe there is a raw_read_seqcount() functionality that is safe for
PREEMPT_RT. Have you looked into using that?

-- Steve