linux-kernel - Re: [PATCH 2/2] perf unwind: fix libunwind when tid != pid

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Message-ID: <20190802133039.GE27223@krava>
Date:   Fri, 2 Aug 2019 15:30:39 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     John Keeping <john@...anate.com>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] perf unwind: fix libunwind when tid != pid

On Mon, Jul 29, 2019 at 06:24:30PM +0100, John Keeping wrote:
> Commit e5adfc3e7e77 ("perf map: Synthesize maps only for thread group
> leader") changed the recording side so that we no longer get mmap events
> for threads other than the thread group leader.
> 
> When a file recorded after this change is loaded, the lack of mmap
> records mean that unwinding is not set up for any other threads.

sry I dont' follow what's the problem here, could you please
describe the scenrio where the current code is failing in
more details

> 
> Following the rationale in that commit, move the libunwind fields into
> struct map_groups and update the libunwind functions to take this
> instead of the struct thread.  This is only required for
> unwind__finish_access which must now be called from map_groups__delete
> and the others are changed for symmetry.
> 
> Note that unwind__get_entries keeps the thread argument since it is
> required for symbol lookup and the libdw unwind provider uses the thread
> ID.

SNIP

> @@ -59,37 +59,31 @@ int unwind__prepare_access(struct thread *thread, struct map *map,
>  		return 0;
>  	}
>  out_register:
> -	unwind__register_ops(thread, ops);
> +	unwind__register_ops(mg, ops);
>  
> -	err = thread->unwind_libunwind_ops->prepare_access(thread);
> +	err = mg->unwind_libunwind_ops->prepare_access(mg);
>  	if (initialized)
>  		*initialized = err ? false : true;
>  	return err;
>  }
>  
> -void unwind__flush_access(struct thread *thread)
> +void unwind__flush_access(struct map_groups *mg)
>  {
> -	if (!dwarf_callchain_users)
> -		return;

why did you remove this check?

> -
> -	if (thread->unwind_libunwind_ops)
> -		thread->unwind_libunwind_ops->flush_access(thread);
> +	if (mg->unwind_libunwind_ops)
> +		mg->unwind_libunwind_ops->flush_access(mg);
>  }
>  
> -void unwind__finish_access(struct thread *thread)
> +void unwind__finish_access(struct map_groups *mg)
>  {
> -	if (!dwarf_callchain_users)
> -		return;

why did you remove this check?

thanks,
jirka