linux-kernel - Re: [PATCH v10 04/14] unwind_user/deferred: Add unwind_deferred

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Message-ID: <20250618112355.47ed62e6@gandalf.local.home>
Date: Wed, 18 Jun 2025 11:23:55 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.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 v10 04/14] unwind_user/deferred: Add
 unwind_deferred_trace()

On Wed, 18 Jun 2025 16:01:11 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Jun 10, 2025 at 08:54:25PM -0400, Steven Rostedt wrote:
> 
> > +#define UNWIND_MAX_ENTRIES 512  
> 
> The reason this is 512 is so that you end up with a whole page below?

Possibly. We could probably even make that configurable. Perhaps just use
sysctl_perf_event_max_contexts_per_stack ?

Josh, any comments about why you picked this number?

-- Steve

> 
> > +int unwind_deferred_trace(struct unwind_stacktrace *trace)
> > +{
> > +	struct unwind_task_info *info = &current->unwind_info;
> > +
> > +	/* Should always be called from faultable context */
> > +	might_fault();
> > +
> > +	if (current->flags & PF_EXITING)
> > +		return -EINVAL;
> > +
> > +	if (!info->entries) {
> > +		info->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
> > +					      GFP_KERNEL);  
>