Re: [PATCH] perf cs-etm: Remove duplicate and incorrect aux size checks

[Leo Yan <leo.yan@linaro.org>]

Hi James,

On Wed, Dec 08, 2021 at 11:54:35AM +0000, James Clark wrote:
> There are two checks, one is for size when running without admin, but
> this one is covered by the driver and reported on in more detail here
> (builtin-record.c):
> 
>   pr_err("Permission error mapping pages.\n"
>          "Consider increasing "
>          "/proc/sys/kernel/perf_event_mlock_kb,\n"
>          "or try again with a smaller value of -m/--mmap_pages.\n"
>          "(current value: %u,%u)\n",

I looked into the kernel code and found:

  sysctl_perf_event_mlock = 512 + (PAGE_SIZE / 1024);  // 512KB + 1 page

If the system have multiple cores, let's say 8 cores, then kernel even
can relax the limitaion with:

  user_lock_limit *= num_online_cpus();

So means the memory lock limitation is:

  (512KB + 1 page) * 8 = 4MB + 8 pages.

Seems to me, it's much relax than the user space's limitaion 128KB.
And let's imagine for Arm server, the permitted buffer size can be a
huge value (e.g. for a system with 128 cores).

Could you confirm if this is right?

Thanks,
Leo

> This had the effect of artificially limiting the aux buffer size to a
> value smaller than what was allowed because perf_event_mlock_kb wasn't
> taken into account.
> 
> The second is to check for a power of two, but this is covered here
> (evlist.c):
> 
>   pr_info("rounding mmap pages size to %s (%lu pages)\n",
>           buf, pages);
> 
> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>  tools/perf/arch/arm/util/cs-etm.c | 19 -------------------
>  1 file changed, 19 deletions(-)
> 
> diff --git a/tools/perf/arch/arm/util/cs-etm.c b/tools/perf/arch/arm/util/cs-etm.c
> index 293a23bf8be3..8a3d54a86c9c 100644
> --- a/tools/perf/arch/arm/util/cs-etm.c
> +++ b/tools/perf/arch/arm/util/cs-etm.c
> @@ -407,25 +407,6 @@ static int cs_etm_recording_options(struct auxtrace_record *itr,
>  
>  	}
>  
> -	/* Validate auxtrace_mmap_pages provided by user */
> -	if (opts->auxtrace_mmap_pages) {
> -		unsigned int max_page = (KiB(128) / page_size);
> -		size_t sz = opts->auxtrace_mmap_pages * (size_t)page_size;
> -
> -		if (!privileged &&
> -		    opts->auxtrace_mmap_pages > max_page) {
> -			opts->auxtrace_mmap_pages = max_page;
> -			pr_err("auxtrace too big, truncating to %d\n",
> -			       max_page);
> -		}
> -
> -		if (!is_power_of_2(sz)) {
> -			pr_err("Invalid mmap size for %s: must be a power of 2\n",
> -			       CORESIGHT_ETM_PMU_NAME);
> -			return -EINVAL;
> -		}
> -	}
> -
>  	if (opts->auxtrace_snapshot_mode)
>  		pr_debug2("%s snapshot size: %zu\n", CORESIGHT_ETM_PMU_NAME,
>  			  opts->auxtrace_snapshot_size);
> -- 
> 2.28.0
>