Re: [PATCH] tools: perf: allow opt-in "no-dso." files in /dev/shm

[Stephen Brennan]

Hello,

A friendly ping on this patch. I know it's a bit niche but it would
resolve some real user issues with how perf handles POSIX shared memory
regions.

Thanks,
Stephen

Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> writes:
> Applications can place JIT-compiled code in POSIX shared memory so that
> multiple processes can execute the same code. On Linux, these shared
> memory objects appear as file mappings under /dev/shm.
>
> Perf uses /tmp/perf-$pid.map to resolve symbols for executable SysV
> shared memory mappings, but it treats /dev/shm mappings as ordinary
> DSOs. As a result, applications which provide perf-map symbols for JIT
> code in POSIX shared memory don't get those symbols resolved, while they
> do with SysV shared memory.
>
> It would be incorrect to classify every /dev/shm mapping as no-DSO
> memory though, because /dev/shm is just a tmpfs mount, and it can
> contain ELF files.
>
> Add a filename prefix "/dev/shm/no-dso." which is recognized by
> is_no_dso_memory(). Applications can opt-in by naming their shared
> memory objects "no-dso.<name>". Executable mappings with this prefix
> will use the perf-map file, while other /dev/shm mappings will keep the
> existing behavior. This allows applications that already emit perf maps
> to use POSIX shared memory for JIT code, without needing to implement
> jitdump support.
>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
> ---
>  tools/perf/util/map.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/tools/perf/util/map.h b/tools/perf/util/map.h
> index fb0279810ae99..c2e31be232c88 100644
> --- a/tools/perf/util/map.h
> +++ b/tools/perf/util/map.h
> @@ -258,6 +258,8 @@ static inline int is_no_dso_memory(const char *filename)
>  {
>  	return !strncmp(filename, "[stack", 6) ||
>  	       !strncmp(filename, "/SYSV", 5)  ||
> +	       !strncmp(filename, "/dev/shm/no-dso.",
> +	                sizeof("/dev/shm/no-dso.") - 1) ||
>  	       !strcmp(filename, "[heap]");
>  }
>  
> -- 
> 2.52.0