linux-kernel - Re: [PATCH v3] trace/pid_list: optimize pid

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Message-ID: <20251113160739.DUvh9i_o@linutronix.de>
Date: Thu, 13 Nov 2025 17:07:39 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Yongliang Gao <leonylgao@...il.com>, mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, frankjpliu@...cent.com,
	Yongliang Gao <leonylgao@...cent.com>,
	Huang Cun <cunhuang@...cent.com>
Subject: Re: [PATCH v3] trace/pid_list: optimize pid_list->lock contention

On 2025-11-13 10:51:06 [-0500], Steven Rostedt wrote:
> Yes, because they are only tested in sched_switch and fork and exit tracepoints.
> 
> Although, this was written when tracepoint callbacks were always called
> under preempt disable. Perhaps we need to change that call to:
> 
> 	tracepoint_synchronize_unregister()
> 
> Or add a preempt_disable() around the callers.

Please don't. Please do a regular rcu_read_lock() ;)
I tried to get rid of the preempt_disable() around tracepoints so that
the attached BPF callbacks are not invoked with disabled preemption. I
haven't followed up here in a while but I think Paul's SRCU work goes
in the right direction.

> I'm very nervous about using RCU here. It will add a lot more corner cases
> that needs to be accounted for. The complexity doesn't appear to be worth
> it. I'd rather just keep the raw spin locks than to convert it to RCU.
> 
> The seqcount makes sense to me. It's simple and keeps the same paradigm as
> what we have. What's wrong with using it?

I'm fine with it once you explained under what conditions retry can
happen.  Thank you.

> -- Steve

Sebastian