Re: [LKP] Re: [x86/mce] 1de08dccd3: will-it-scale.per_process_ops -14.1% regression

[Feng Tang]

On Mon, Aug 31, 2020 at 09:55:17AM +0100, Mel Gorman wrote:
> On Mon, Aug 31, 2020 at 04:23:06PM +0800, Feng Tang wrote:
> > On Mon, Aug 31, 2020 at 08:56:11AM +0100, Mel Gorman wrote:
> > > On Mon, Aug 31, 2020 at 10:16:38AM +0800, Feng Tang wrote:
> > > > > So why don't you define both variables with DEFINE_PER_CPU_ALIGNED and
> > > > > check if all your bad measurements go away this way?
> > > > 
> > > > For 'arch_freq_scale', there are other percpu variables in the same
> > > > smpboot.c: 'arch_prev_aperf' and 'arch_prev_mperf', and in hot path
> > > > arch_scale_freq_tick(), these 3 variables are all accessed, so I didn't 
> > > > touch it. Or maybe we can align the first of these 3 variables, so
> > > > that they sit in one cacheline.
> > > > 
> > > > > You'd also need to check whether there's no detrimental effect from
> > > > > this change on other, i.e., !KNL platforms, and I think there won't
> > > > > be because both variables will be in separate cachelines then and all
> > > > > should be good.
> > > > 
> > > > Yes, these kind of changes should be verified on other platforms.
> > > > 
> > > > One thing still puzzles me, that the 2 variables are per-cpu things, and
> > > > there is no case of many CPU contending, why the cacheline layout matters?
> > > > I doubt it is due to the contention of the same cache set, and am trying
> > > > to find some way to test it.
> > > > 
> > > 
> > > Because if you have two structures that are per-cpu and not cache-aligned
> > > then a write in one can bounce the cache line in another due to
> > > cache coherency protocol. It's generally called "false cache line
> > > sharing". https://en.wikipedia.org/wiki/False_sharing has basic examples
> > > (lets not get into whether wikipedia is a valid citation source, there
> > > are books on the topic if someone really cared).
> > 
> > For 'arch_freq_scale' and 'tsc_adjust' percpu variable, they are only
> > accessed by their own CPU, and usually no other CPU will touch them,
> 
> Read "false sharing again". Two adjacent per-CPU structures can still
> interfere with each other if the structures happen to cross a cache line
> boundary and are not cache aligned.
 
Sure, will recheck that wiki, thanks.

Some cache info about the test box is, it's the Xeon Phi platform with
72 cores, and each core has 4 HT threads, so there are 288 CPUs. The L1
D-cache and I-cache are both 32KB. There is no L3 cache, and every 2
cores share one 1MB L2 cache. The L1 D-cache is 64 sets, 8 ways, and the
L2 cache is 1024 sets, 16 ways.

Thanks,
Feng

> > the
> > hot node path only use this_cpu_read/write/ptr. And each CPU's static
> > percpu variables are all packed together in one area (256KB for one CPU on
> > this test box),
> 
> If the structure is not cache aligned (probably 64KB) then there is a
> boundary when cache line bounces can occur.
> 
> -- 
> Mel Gorman
> SUSE Labs