linux-kernel - Re: [PATCH v11 03/11] perf: Use current->flags & PF

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Message-ID: <a3b456f2-deeb-45c9-b509-23bbe5e96cfd@kernel.dk>
Date: Thu, 26 Jun 2025 07:48:40 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, x86@...nel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Andrii Nakryiko <andrii@...nel.org>, Indu Bhagat <indu.bhagat@...cle.com>,
 "Jose E. Marchesi" <jemarch@....org>,
 Beau Belgrave <beaub@...ux.microsoft.com>, Jens Remus
 <jremus@...ux.ibm.com>, Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v11 03/11] perf: Use current->flags & PF_KTHREAD instead
 of current->mm == NULL

On 6/25/25 5:15 PM, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> To determine if a task is a kernel thread or not, it is more reliable to
> use (current->flags & PF_KTHREAD) than to rely on current->mm being NULL.
> That is because some kernel tasks (io_uring helpers) may have a mm field.

This commit message is very odd, imho, and wrong. To check if it's a
kernel thread yes you should use PF_KTHREAD, but that has nothing to do
with PF_USER_WORKER. In fact, as mentioned in a previous reply,
current->mm may be non-NULL for a kthread as well, if it's done
kthread_use_mm().

If the current check for "is kernel thread" was using ->mm to gauge
then, then the current check was just wrong, period.

-- 
Jens Axboe