[PATCH 1/3] perf lock contention: Handle error in a single place

[PATCH 1/3] perf lock contention: Handle error in a single place

[Namhyung Kim]

It has some duplicate codes to do the same job.  Let's add a label and
goto there to handle errors in a single place.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index d931a898c434..e8a6f6463019 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -439,11 +439,8 @@ int contention_end(u64 *ctx)
 
 	duration = bpf_ktime_get_ns() - pelem->timestamp;
 	if ((__s64)duration < 0) {
-		pelem->lock = 0;
-		if (need_delete)
-			bpf_map_delete_elem(&tstamp, &pid);
 		__sync_fetch_and_add(&time_fail, 1);
-		return 0;
+		goto out;
 	}
 
 	switch (aggr_mode) {
@@ -477,11 +474,8 @@ int contention_end(u64 *ctx)
 	data = bpf_map_lookup_elem(&lock_stat, &key);
 	if (!data) {
 		if (data_map_full) {
-			pelem->lock = 0;
-			if (need_delete)
-				bpf_map_delete_elem(&tstamp, &pid);
 			__sync_fetch_and_add(&data_fail, 1);
-			return 0;
+			goto out;
 		}
 
 		struct contention_data first = {
@@ -502,10 +496,7 @@ int contention_end(u64 *ctx)
 				data_map_full = 1;
 			__sync_fetch_and_add(&data_fail, 1);
 		}
-		pelem->lock = 0;
-		if (need_delete)
-			bpf_map_delete_elem(&tstamp, &pid);
-		return 0;
+		goto out;
 	}
 
 	__sync_fetch_and_add(&data->total_time, duration);
@@ -517,6 +508,7 @@ int contention_end(u64 *ctx)
 	if (data->min_time > duration)
 		data->min_time = duration;
 
+out:
 	pelem->lock = 0;
 	if (need_delete)
 		bpf_map_delete_elem(&tstamp, &pid);
-- 
2.46.0.469.g59c65b2a67-goog

[PATCH 2/3] perf lock contention: Simplify spinlock check

[Namhyung Kim]

The LCB_F_SPIN bit is used for spinlock, rwlock and optimistic spinning
in mutex.  In get_tstamp_elem() it needs to check spinlock and rwlock
only.  As mutex sets the LCB_F_MUTEX, it can check those two bits and
reduce the number of operations.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index e8a6f6463019..4b7237e178bd 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -323,8 +323,7 @@ static inline struct tstamp_data *get_tstamp_elem(__u32 flags)
 	struct tstamp_data *pelem;
 
 	/* Use per-cpu array map for spinlock and rwlock */
-	if (flags == (LCB_F_SPIN | LCB_F_READ) || flags == LCB_F_SPIN ||
-	    flags == (LCB_F_SPIN | LCB_F_WRITE)) {
+	if ((flags & (LCB_F_SPIN | LCB_F_MUTEX)) == LCB_F_SPIN) {
 		__u32 idx = 0;
 
 		pelem = bpf_map_lookup_elem(&tstamp_cpu, &idx);
-- 
2.46.0.469.g59c65b2a67-goog

[PATCH 3/3] perf lock contention: Do not fail EEXIST for update

[Namhyung Kim]

When it updates the lock stat for the first time, it needs to create an
element in the BPF hash map.  But if there's a concurrent thread waiting
for the same lock (like for rwsem or rwlock), it might race with the
thread and possibly failed to update with -EEXIST.  In that case, it can
lookup the map again and put the data there instead of failing.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_skel/lock_contention.bpf.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
index 4b7237e178bd..52a876b42699 100644
--- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
+++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
@@ -491,6 +491,12 @@ int contention_end(u64 *ctx)
 
 		err = bpf_map_update_elem(&lock_stat, &key, &first, BPF_NOEXIST);
 		if (err < 0) {
+			if (err == -EEXIST) {
+				/* it lost the race, try to get it again */
+				data = bpf_map_lookup_elem(&lock_stat, &key);
+				if (data != NULL)
+					goto found;
+			}
 			if (err == -E2BIG)
 				data_map_full = 1;
 			__sync_fetch_and_add(&data_fail, 1);
@@ -498,6 +504,7 @@ int contention_end(u64 *ctx)
 		goto out;
 	}
 
+found:
 	__sync_fetch_and_add(&data->total_time, duration);
 	__sync_fetch_and_add(&data->count, 1);
 
-- 
2.46.0.469.g59c65b2a67-goog

Re: [PATCH 1/3] perf lock contention: Handle error in a single place

[Arnaldo Carvalho de Melo]

On Thu, Aug 29, 2024 at 11:51:48PM -0700, Namhyung Kim wrote:
> It has some duplicate codes to do the same job.  Let's add a label and
> goto there to handle errors in a single place.

Thanks, applied to perf-tools-next,

- Arnaldo
 
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/bpf_skel/lock_contention.bpf.c | 16 ++++------------
>  1 file changed, 4 insertions(+), 12 deletions(-)
> 
> diff --git a/tools/perf/util/bpf_skel/lock_contention.bpf.c b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> index d931a898c434..e8a6f6463019 100644
> --- a/tools/perf/util/bpf_skel/lock_contention.bpf.c
> +++ b/tools/perf/util/bpf_skel/lock_contention.bpf.c
> @@ -439,11 +439,8 @@ int contention_end(u64 *ctx)
>  
>  	duration = bpf_ktime_get_ns() - pelem->timestamp;
>  	if ((__s64)duration < 0) {
> -		pelem->lock = 0;
> -		if (need_delete)
> -			bpf_map_delete_elem(&tstamp, &pid);
>  		__sync_fetch_and_add(&time_fail, 1);
> -		return 0;
> +		goto out;
>  	}
>  
>  	switch (aggr_mode) {
> @@ -477,11 +474,8 @@ int contention_end(u64 *ctx)
>  	data = bpf_map_lookup_elem(&lock_stat, &key);
>  	if (!data) {
>  		if (data_map_full) {
> -			pelem->lock = 0;
> -			if (need_delete)
> -				bpf_map_delete_elem(&tstamp, &pid);
>  			__sync_fetch_and_add(&data_fail, 1);
> -			return 0;
> +			goto out;
>  		}
>  
>  		struct contention_data first = {
> @@ -502,10 +496,7 @@ int contention_end(u64 *ctx)
>  				data_map_full = 1;
>  			__sync_fetch_and_add(&data_fail, 1);
>  		}
> -		pelem->lock = 0;
> -		if (need_delete)
> -			bpf_map_delete_elem(&tstamp, &pid);
> -		return 0;
> +		goto out;
>  	}
>  
>  	__sync_fetch_and_add(&data->total_time, duration);
> @@ -517,6 +508,7 @@ int contention_end(u64 *ctx)
>  	if (data->min_time > duration)
>  		data->min_time = duration;
>  
> +out:
>  	pelem->lock = 0;
>  	if (need_delete)
>  		bpf_map_delete_elem(&tstamp, &pid);
> -- 
> 2.46.0.469.g59c65b2a67-goog