Re: [PATCH 2/2] [gdb] Exclude unavailable hardware threads in default worker-threads

[Thiago Jung Bauermann <thiago.bauermann@linaro.org>]

Tom de Vries <tdevries@suse.de> writes:

> The default worker-threads is set to the number of hardware threads:
> ...
> $ gdb -q -batch -ex "maint show worker-thread"
> The number of worker threads GDB can use is the default (currently 8).
> ...
> but does not take into account how many of those are actually available.
>
> If we reduce the amount of available threads to 1, we get the same answer:
> ...
> $ taskset -c 0 gdb -q -batch -ex "maint show worker-thread"
> The number of worker threads GDB can use is the default (currently 8).
> ...
>
> Fix this on linux using sched_getaffinity:
> ...
> $ taskset -c 0 gdb -q -batch -ex "maint show worker-thread"
> The number of worker threads GDB can use is the default (currently 1).
> $ gdb -q -batch -ex "maint show worker-thread"
> The number of worker threads GDB can use is the default (currently 8).
> ...
> ---
>  gdb/maint.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)

This is a nice improvement. At least some good came out of my red
herring. Thanks!

One comment, but:

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>

> diff --git a/gdb/maint.c b/gdb/maint.c
> index 1daa6ce1d2e..fbddfe15591 100644
> --- a/gdb/maint.c
> +++ b/gdb/maint.c
> @@ -47,6 +47,10 @@
>  #include "cli/cli-style.h"
>  #include "cli/cli-cmds.h"
>  
> +#ifdef __linux__
> +#include <sched.h>

The sched_getaffinity(2) man page mentions that one should define
_GNU_SOURCE before including <sched.h>

> +#endif
> +
>  static void maintenance_do_deprecate (const char *, int);
>  
>  #ifndef _WIN32
> @@ -882,6 +886,20 @@ update_thread_pool_size ()
>  	 systems.  */
>        const int max_thread_count = 8;
>        n_threads = std::min (hardware_threads, max_thread_count);
> +
> +      if (n_threads > 1)
> +	{
> +	  /* Exclude unavailable hardware threads (for instance made
> +	     unavailable using taskset).  */
> +#ifdef __linux__
> +	  cpu_set_t cpus;
> +	  int res = sched_getaffinity (getpid (), sizeof (cpu_set_t), &cpus);
> +	  if (res == 0)
> +	    n_threads = std::min (n_threads, CPU_COUNT (&cpus));
> +#else
> +	  /* Todo: handle other platforms.  */
> +#endif
> +	}
>      }
>  
>    gdb::thread_pool::g_thread_pool->set_thread_count (n_threads);

-- 
Thiago
(he/him)