linux-kernel - Re: [PATCH] LoongArch: Fix PMU counter allocation for mixed-type event groups

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Message-ID: <7214053a-070c-45c3-8cd8-b4306c49328a@linux.intel.com>
Date: Sun, 4 Jan 2026 12:58:12 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Lisa Robinson <lisa@...efly.space>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Huacai Chen <chenhuacai@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, James Clark
 <james.clark@...aro.org>, WANG Xuerui <kernel@...0n.name>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 loongarch@...ts.linux.dev
Subject: Re: [PATCH] LoongArch: Fix PMU counter allocation for mixed-type
 event groups


On 12/31/2025 5:15 PM, Lisa Robinson wrote:
> When validating a perf event group, validate_group() unconditionally
> attempts to allocate hardware PMU counters for the leader, sibling
> events and the new event being added.
>
> This is incorrect for mixed-type groups. If a PERF_TYPE_SOFTWARE event
> ispart of the group, the current code still tries to allocate a hardware
> PMU counter for it, which can wrongly consume hardware PMU resources and
> cause spurious allocation failures.
>
> Fix this by only allocating PMU counters for hardware events during group
> validation, and skipping software events.
>
> A trimmed down reproducer is as simple as this:
>
>   #include <stdio.h>
>   #include <assert.h>
>   #include <unistd.h>
>   #include <string.h>
>   #include <sys/syscall.h>
>   #include <linux/perf_event.h>
>
>   int
>   main (int argc, char *argv[])
>   {
>   	struct perf_event_attr attr = { 0 };
>   	int fds[5];
>
>   	attr.disabled = 1;
>   	attr.exclude_kernel = 1;
>   	attr.exclude_hv = 1;
>   	attr.read_format = PERF_FORMAT_TOTAL_TIME_ENABLED |
>   		PERF_FORMAT_TOTAL_TIME_RUNNING | PERF_FORMAT_ID | PERF_FORMAT_GROUP;
>   	attr.size = sizeof (attr);
>
>   	attr.type = PERF_TYPE_SOFTWARE;
>   	attr.config = PERF_COUNT_SW_DUMMY;
>   	fds[0] = syscall (SYS_perf_event_open, &attr, 0, -1, -1, 0);
>   	assert (fds[0] >= 0);
>
>   	attr.type = PERF_TYPE_HARDWARE;
>   	attr.config = PERF_COUNT_HW_CPU_CYCLES;
>   	fds[1] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
>   	assert (fds[1] >= 0);
>
>   	attr.type = PERF_TYPE_HARDWARE;
>   	attr.config = PERF_COUNT_HW_INSTRUCTIONS;
>   	fds[2] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
>   	assert (fds[2] >= 0);
>
>   	attr.type = PERF_TYPE_HARDWARE;
>   	attr.config = PERF_COUNT_HW_BRANCH_MISSES;
>   	fds[3] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
>   	assert (fds[3] >= 0);
>
>   	attr.type = PERF_TYPE_HARDWARE;
>   	attr.config = PERF_COUNT_HW_CACHE_REFERENCES;
>   	fds[4] = syscall (SYS_perf_event_open, &attr, 0, -1, fds[0], 0);
>   	assert (fds[4] >= 0);
>
>   	printf ("PASSED\n");
>
>   	return 0;
>   }
>
> Fixes: b37042b2bb7c ("LoongArch: Add perf events support")
> Signed-off-by: Lisa Robinson <lisa@...efly.space>
> ---
>  arch/loongarch/kernel/perf_event.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/arch/loongarch/kernel/perf_event.c b/arch/loongarch/kernel/perf_event.c
> index 9d257c8519c9..82cc08c2b056 100644
> --- a/arch/loongarch/kernel/perf_event.c
> +++ b/arch/loongarch/kernel/perf_event.c
> @@ -633,15 +633,24 @@ static int validate_group(struct perf_event *event)
>  
>  	memset(&fake_cpuc, 0, sizeof(fake_cpuc));
>  
> -	if (loongarch_pmu_alloc_counter(&fake_cpuc, &leader->hw) < 0)
> +	if ((PERF_TYPE_HARDWARE == leader->attr.type ||
> +	     PERF_TYPE_HW_CACHE == leader->attr.type ||
> +	     PERF_TYPE_RAW == leader->attr.type) &&

Better introduce an inline function to check these event types instead of
duplicating same code.


> +	    loongarch_pmu_alloc_counter(&fake_cpuc, &leader->hw) < 0)
>  		return -EINVAL;
>  
>  	for_each_sibling_event(sibling, leader) {
> -		if (loongarch_pmu_alloc_counter(&fake_cpuc, &sibling->hw) < 0)
> +		if ((PERF_TYPE_HARDWARE == sibling->attr.type ||
> +		     PERF_TYPE_HW_CACHE == sibling->attr.type ||
> +		     PERF_TYPE_RAW == sibling->attr.type) &&
> +		    loongarch_pmu_alloc_counter(&fake_cpuc, &sibling->hw) < 0)
>  			return -EINVAL;
>  	}
>  
> -	if (loongarch_pmu_alloc_counter(&fake_cpuc, &event->hw) < 0)
> +	if ((PERF_TYPE_HARDWARE == event->attr.type ||
> +	     PERF_TYPE_HW_CACHE == event->attr.type ||
> +	     PERF_TYPE_RAW == event->attr.type) &&
> +	    loongarch_pmu_alloc_counter(&fake_cpuc, &event->hw) < 0)
>  		return -EINVAL;
>  
>  	return 0;