Re: [PATCH v11 19/19] futex: Allow to make the private hash immutable.

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Sebastian.

On 4/7/25 21:27, Sebastian Andrzej Siewior wrote:
> My initial testing showed that
> 	perf bench futex hash
> 
> reported less operations/sec with private hash. After using the same
> amount of buckets in the private hash as used by the global hash then
> the operations/sec were about the same.
> 
> This changed once the private hash became resizable. This feature added
> a RCU section and reference counting via atomic inc+dec operation into
> the hot path.
> The reference counting can be avoided if the private hash is made
> immutable.
> Extend PR_FUTEX_HASH_SET_SLOTS by a fourth argument which denotes if the
> private should be made immutable. Once set (to true) the a further
> resize is not allowed (same if set to global hash).
> Add PR_FUTEX_HASH_GET_IMMUTABLE which returns true if the hash can not
> be changed.
> Update "perf bench" suite.
> 

It would be good option for the application to decide if it needs this.
Using this option makes the perf regression goes away using previous number of buckets.

Acked-by: Shrikanth Hegde <sshegde@linux.ibm.com>

base:
./perf bench futex hash
Averaged 1556023 operations/sec (+- 0.08%), total secs = 10  <<-- 1.5M

with series:
./perf bench futex hash -b32768
Averaged 126499 operations/sec (+- 0.41%), total secs = 10   <<-- .12M

./perf bench futex hash -Ib32768
Averaged 1549339 operations/sec (+- 0.08%), total secs = 10  <<-- 1.5M


> For comparison, results of "perf bench futex hash -s":
> - Xeon CPU E5-2650, 2 NUMA nodes, total 32 CPUs:
>    - Before the introducing task local hash
>      shared  Averaged 1.487.148 operations/sec (+- 0,53%), total secs = 10
>      private Averaged 2.192.405 operations/sec (+- 0,07%), total secs = 10
> 
>    - With the series
>      shared  Averaged 1.326.342 operations/sec (+- 0,41%), total secs = 10
>      -b128   Averaged   141.394 operations/sec (+- 1,15%), total secs = 10
>      -Ib128  Averaged   851.490 operations/sec (+- 0,67%), total secs = 10
>      -b8192  Averaged   131.321 operations/sec (+- 2,13%), total secs = 10
>      -Ib8192 Averaged 1.923.077 operations/sec (+- 0,61%), total secs = 10
>      128 is the default allocation of hash buckets.
>      8192 was the previous amount of allocated hash buckets.
> 
> - Xeon(R) CPU E7-8890 v3, 4 NUMA nodes, total 144 CPUs:
>    - Before the introducing task local hash
>      shared   Averaged 1.810.936 operations/sec (+- 0,26%), total secs = 20
>      private  Averaged 2.505.801 operations/sec (+- 0,05%), total secs = 20
> 
>    - With the series
>      shared   Averaged 1.589.002 operations/sec (+- 0,25%), total secs = 20
>      -b1024   Averaged    42.410 operations/sec (+- 0,20%), total secs = 20
>      -Ib1024  Averaged   740.638 operations/sec (+- 1,51%), total secs = 20
>      -b65536  Averaged    48.811 operations/sec (+- 1,35%), total secs = 20
>      -Ib65536 Averaged 1.963.165 operations/sec (+- 0,18%), total secs = 20
>      1024 is the default allocation of hash buckets.
>      65536 was the previous amount of allocated hash buckets.
> 
> Cc: "Liang, Kan" <kan.liang@linux.intel.com>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Ian Rogers <irogers@google.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: linux-perf-users@vger.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>   include/linux/futex.h                  |  2 +-
>   include/uapi/linux/prctl.h             |  1 +
>   kernel/futex/core.c                    | 42 ++++++++++++++++++++++----
>   kernel/sys.c                           |  2 +-
>   tools/include/uapi/linux/prctl.h       |  1 +
>   tools/perf/bench/futex-hash.c          |  1 +
>   tools/perf/bench/futex-lock-pi.c       |  1 +
>   tools/perf/bench/futex-requeue.c       |  1 +
>   tools/perf/bench/futex-wake-parallel.c |  1 +
>   tools/perf/bench/futex-wake.c          |  1 +
>   tools/perf/bench/futex.c               |  8 +++--
>   tools/perf/bench/futex.h               |  1 +
>   12 files changed, 51 insertions(+), 11 deletions(-)
> 

nit: Does it makes sense to split this patch into futex and perf?