Re: [PATCH] rtla: Define _GNU_SOURCE in timerlat_bpf.c

[John Kacur <jkacur@redhat.com>]

On Wed, 30 Apr 2025, Tomas Glozar wrote:

> Newer versions of glibc include a definition of struct sched_attr in
> bits/sched.h (included through sched.h which is included by rtla).
> Commit 0eecee340672 ("tools/rtla: fix collision with glibc
> sched_attr/sched_set_attr") has modified the definition of struct
> sched_attr in utils.h, so that it is only applied with older versions of
> glibc that do not define it, in order to prevent build failure.
> 
> The definition in bits/sched.h depends on _GNU_SOURCE.
> timerlat_bpf.c does not define _GNU_SOURCE, making it fall back to the
> definition in utils.h. The latter has two fields less, leading to
> shifted offsets of struct timerlat_params in timerlat_bpf_init.
> 
> Because of the shift, timerlat_bpf_init incorrectly reads
> params->entries as 0 for timerlat-hist and disables the creation of
> histogram maps, causing breakage in BPF sample collection mode:
> 
> $ rtla timerlat hist -d 1s
> Error pulling BPF data
> 
> Fix the issue by also defining _GNU_SOURCE in timerlat_bpf.c.
> 
> Fixes: e34293ddcebd ("rtla/timerlat: Add BPF skeleton to collect samples")
> Signed-off-by: Tomas Glozar <tglozar@redhat.com>
> ---
>  tools/tracing/rtla/src/timerlat_bpf.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/tools/tracing/rtla/src/timerlat_bpf.c b/tools/tracing/rtla/src/timerlat_bpf.c
> index 5abee884037a..0bc44ce5d69b 100644
> --- a/tools/tracing/rtla/src/timerlat_bpf.c
> +++ b/tools/tracing/rtla/src/timerlat_bpf.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #ifdef HAVE_BPF_SKEL
> +#define _GNU_SOURCE
>  #include "timerlat.h"
>  #include "timerlat_bpf.h"
>  #include "timerlat.skel.h"
> -- 

This works, and we need it, so
Reviewed-by: John Kacur <jkacur@redhat.com>
--
Question for Tomas, should we spend sometime to make this work for older 
versions of glibc without a definition of struct sched_attr?