linux-kernel - Re: [PATCH] powerpc/perf: prevent mixed EBB and non-EBB events

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Message-Id: <F791C89D-E644-43D3-8229-3618AF7DE2C2@linux.vnet.ibm.com>
Date:   Fri, 5 Mar 2021 11:20:25 +0530
From:   Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
To:     Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/perf: prevent mixed EBB and non-EBB events



> On 24-Feb-2021, at 5:51 PM, Thadeu Lima de Souza Cascardo <cascardo@...onical.com> wrote:
> 
> EBB events must be under exclusive groups, so there is no mix of EBB and
> non-EBB events on the same PMU. This requirement worked fine as perf core
> would not allow other pinned events to be scheduled together with exclusive
> events.
> 
> This assumption was broken by commit 1908dc911792 ("perf: Tweak
> perf_event_attr::exclusive semantics").
> 
> After that, the test cpu_event_pinned_vs_ebb_test started succeeding after
> read_events, but worse, the task would not have given access to PMC1, so
> when it tried to write to it, it was killed with "illegal instruction".
> 
> Preventing mixed EBB and non-EBB events from being add to the same PMU will
> just revert to the previous behavior and the test will succeed.


Hi,

Thanks for checking this. I checked your patch which is fixing “check_excludes” to make
sure all events must agree on EBB. But in the PMU group constraints, we already have check for
EBB events. This is in arch/powerpc/perf/isa207-common.c ( isa207_get_constraint function ).

<<>>
mask  |= CNST_EBB_VAL(ebb);
value |= CNST_EBB_MASK;
<<>>

But the above setting for mask and value is interchanged. We actually need to fix here.

Below patch should fix this:

diff --git a/arch/powerpc/perf/isa207-common.c b/arch/powerpc/perf/isa207-common.c
index e4f577da33d8..8b5eeb6fb2fb 100644
--- a/arch/powerpc/perf/isa207-common.c
+++ b/arch/powerpc/perf/isa207-common.c
@@ -447,8 +447,8 @@ int isa207_get_constraint(u64 event, unsigned long *maskp, unsigned long *valp,
         * EBB events are pinned & exclusive, so this should never actually
         * hit, but we leave it as a fallback in case.
         */
-       mask  |= CNST_EBB_VAL(ebb);
-       value |= CNST_EBB_MASK;
+       mask  |= CNST_EBB_MASK;
+       value |= CNST_EBB_VAL(ebb);
 
        *maskp = mask;
        *valp = value;


Can you please try with this patch.

Thanks
Athira


> 
> Fixes: 1908dc911792 (perf: Tweak perf_event_attr::exclusive semantics)
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
> ---
> arch/powerpc/perf/core-book3s.c | 20 ++++++++++++++++----
> 1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index 43599e671d38..d767f7944f85 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -1010,9 +1010,25 @@ static int check_excludes(struct perf_event **ctrs, unsigned int cflags[],
> 			  int n_prev, int n_new)
> {
> 	int eu = 0, ek = 0, eh = 0;
> +	bool ebb = false;
> 	int i, n, first;
> 	struct perf_event *event;
> 
> +	n = n_prev + n_new;
> +	if (n <= 1)
> +		return 0;
> +
> +	first = 1;
> +	for (i = 0; i < n; ++i) {
> +		event = ctrs[i];
> +		if (first) {
> +			ebb = is_ebb_event(event);
> +			first = 0;
> +		} else if (is_ebb_event(event) != ebb) {
> +			return -EAGAIN;
> +		}
> +	}
> +
> 	/*
> 	 * If the PMU we're on supports per event exclude settings then we
> 	 * don't need to do any of this logic. NB. This assumes no PMU has both
> @@ -1021,10 +1037,6 @@ static int check_excludes(struct perf_event **ctrs, unsigned int cflags[],
> 	if (ppmu->flags & PPMU_ARCH_207S)
> 		return 0;
> 
> -	n = n_prev + n_new;
> -	if (n <= 1)
> -		return 0;
> -
> 	first = 1;
> 	for (i = 0; i < n; ++i) {
> 		if (cflags[i] & PPMU_LIMITED_PMC_OK) {
> -- 
> 2.27.0
>