linux-kernel - Re: [PATCH v15 03/10] unwind_user/deferred: Add unwind cache

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Message-ID: <a0f9a3ef-f32a-4bff-8ab1-4181ad61780f@linux.ibm.com>
Date: Mon, 28 Jul 2025 17:46:42 +0200
From: Jens Remus <jremus@...ux.ibm.com>
To: Steven Rostedt <rostedt@...nel.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>,
        Arnaldo Carvalho de Melo <acme@...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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>,
        Sam James <sam@...too.org>
Subject: Re: [PATCH v15 03/10] unwind_user/deferred: Add unwind cache

On 25.07.2025 20:55, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> Cache the results of the unwind to ensure the unwind is only performed
> once, even when called by multiple tracers.
> 
> The cache nr_entries gets cleared every time the task exits the kernel.
> When a stacktrace is requested, nr_entries gets set to the number of
> entries in the stacktrace. If another stacktrace is requested, if
> nr_entries is not zero, then it contains the same stacktrace that would be
> retrieved so it is not processed again and the entries is given to the
> caller.
> 
> Co-developed-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Reviewed-by: Jens Remus <jremus@...ux.ibm.com>

> diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c

> +	cache = info->cache;
> +	trace->entries = cache->entries;
> +
> +	if (cache->nr_entries) {
> +		/*
> +		 * The user stack has already been previously unwound in this
> +		 * entry context.  Skip the unwind and use the cache.
> +		 */
> +		trace->nr = cache->nr_entries;
> +		return 0;
> +	}
> +
>  	trace->nr = 0;
> -	trace->entries = info->entries;
>  	unwind_user(trace, UNWIND_MAX_ENTRIES);
>  
> +	cache->nr_entries = trace->nr;
> +

Would the following alternative to above excerpt be easier to read?

	/* Use the cache, if the user stack has already been previously
	 * unwound in this entry context.  If not this will initialize
	 * trace->nr to zero to trigger the unwind now.
	 */
	cache = info->cache;
	trace->nr = cache->nr_entries;
	trace->entries = cache->entries;

	if (!trace->nr) {
		unwind_user(trace, UNWIND_MAX_ENTRIES);
		cache->nr_entries = trace->nr;
	}

>  	return 0;
>  }

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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