Re: [PATCHv4 26/28] x86/vdso: Align VDSO functions by CPU L1 cache line

On Wed, 12 Jun 2019, Dmitry Safonov wrote:

From: Andrei Vagin <redacted>

After performance testing VDSO patches a noticeable 20% regression was
found on gettime_perf selftest with a cold cache.
As it turns to be, before time namespaces introduction, VDSO functions
were quite aligned to cache lines, but adding a new code to adjust
timens offset inside namespace created a small shift and vdso functions
become unaligned on cache lines.

Add align to vdso functions with gcc option to fix performance drop.

Coping the resulting numbers from cover letter:

Hot CPU cache (more gettime_perf.c cycles - the better):
        | before     | CONFIG_TIME_NS=n | host        | inside timens
--------|------------|------------------|-------------|-------------
cycles  | 139887013  | 139453003        | 139899785   | 128792458
diff (%)| 100        | 99.7             | 100         | 92

Why is CONFIG_TIME_NS=n behaving worse than current mainline and
worse than 'host' mode?

Cold cache (lesser tsc per gettime_perf_cold.c cycle - the better):
        | before     | CONFIG_TIME_NS=n | host        | inside timens
--------|------------|------------------|-------------|-------------
tsc     | 6748       | 6718             | 6862        | 12682
diff (%)| 100        | 99.6             | 101.7       | 188

Weird, now CONFIG_TIME_NS=n is better than current mainline and 'host' mode
drops.

Either I'm misreading the numbers or missing something or I'm just confused
as usual :)

Thanks,

	tglx