futex: Optimize futex hash bucket access patterns

futex: Optimize futex hash bucket access patterns

Breno reported significant c2c HITM in a futex hash heavy workload.

It turns out that the hash bucket to private hash table reverse pointer
(futex_hash_bucket::priv) was to blame. Notably when the hash buckets are
heavily contended, the: 'fph = bh->priv;' load in futex_hash() will typically
miss and consequently become quite expensive.

Since this load in particular is quite superfluous, removing it is fairly
straight forward. However, removing it does not in fact achieve anything much.
The pain moves to the next user, notably: futex_hash_put().

Therefore rework the whole private hash refcounting to avoid needing this back
pointer (and removing it). Instead of passing around 'struct futex_hash_bucket
*hb', pass around a new structure that contains it and the related 'struct
futex_private_hash *fph' pointer in tandem.

Funnily this turns out to remove more code than it adds and significantly
improves futex hash performance (as measured by 'perf bench futex hash'):

SKL dual socket 112 threads:

		  Baseline        Patched
  shared (16k)    1571857         1641435         + 4.4%
  autosize (512)   646390          903371         +39.7%
  -b 256           464395          587014         +26.4%
  -b 512           715687          995943         +39.2%
  -b 1024          995085         1396328         +40.3%
  -b 2048         1293114         1668395         +29.0%
  -b 4096         2124438         2240228         + 5.5%

Zen3 dual socket 256 threads:

                  Baseline        Patched
  shared  (16k)   1275840         1381279         + 8.2%
  autosize (512)  1252745         1482179         +18.3%
  -b 256           856274          955455         +11.5%
  -b 512          1267490         1544010         +21.8%
  -b 1024         1424013         1625424         +14.1%
  -b 2048         1505181         1669342         +10.9%
  -b 4096         1465993         1688932         +15.2%

AMD EPYC 9D64 (Zen4, single socket) 176 threads:

                       Baseline       Patched      Delta
  shared (16k)         1,230,599      1,368,655    +11.2%
  autosize (1024)      1,285,440      1,556,946    +21.1%
  -b 256               1,341,471      1,520,303    +13.3%
  -b 512               1,438,330      1,599,319    +11.2%
  -b 1024              1,443,772      1,622,493    +12.4%
  -b 2048              1,472,108      1,643,975    +11.7%
  -b 4096              1,333,098      1,570,897    +17.8%

Reported-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Breno Leitao <leitao@debian.org>
Tested-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260610135510.GB1430057@noisy.programming.kicks-ass.net