Messages in this thread | | | Date | Mon, 31 Aug 2020 10:16:38 +0800 | From | Feng Tang <> | Subject | Re: [LKP] Re: [x86/mce] 1de08dccd3: will-it-scale.per_process_ops -14.1% regression |
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On Fri, Aug 28, 2020 at 07:48:39PM +0200, Borislav Petkov wrote: > On Tue, Aug 25, 2020 at 02:23:05PM +0800, Feng Tang wrote: > > Also one good news is, we seem to identify the 2 key percpu variables > > out of the list mentioned in previous email: > > 'arch_freq_scale' > > 'tsc_adjust' > > > > These 2 variables are accessed in 2 hot call stacks (for this 288 CPU > > Xeon Phi platform): > > > > - arch_freq_scale is accessed in scheduler tick > > arch_scale_freq_tick+0xaf/0xc0 > > scheduler_tick+0x39/0x100 > > update_process_times+0x3c/0x50 > > tick_sched_handle+0x22/0x60 > > tick_sched_timer+0x37/0x70 > > __hrtimer_run_queues+0xfc/0x2a0 > > hrtimer_interrupt+0x122/0x270 > > smp_apic_timer_interrupt+0x6a/0x150 > > apic_timer_interrupt+0xf/0x20 > > > > - tsc_adjust is accessed in idle entrance > > tsc_verify_tsc_adjust+0xeb/0xf0 > > arch_cpu_idle_enter+0xc/0x20 > > do_idle+0x91/0x280 > > cpu_startup_entry+0x19/0x20 > > start_kernel+0x4f4/0x516 > > secondary_startup_64+0xb6/0xc0 > > > > From systemmap file, for bad kernel these 2 sit in one cache line, while > > for good kernel they sit in 2 separate cache lines. > > > > It also explains why it turns from a regression to an improvement with > > updated gcc/kconfig, as the cache line sharing situation is reversed. > > > > The direct patch I can think of is to make 'tsc_adjust' cache aligned > > to separate these 2 'hot' variables. How do you think? > > > > --- a/arch/x86/kernel/tsc_sync.c > > +++ b/arch/x86/kernel/tsc_sync.c > > @@ -29,7 +29,7 @@ struct tsc_adjust { > > bool warned; > > }; > > > > -static DEFINE_PER_CPU(struct tsc_adjust, tsc_adjust); > > +static DEFINE_PER_CPU_ALIGNED(struct tsc_adjust, tsc_adjust); > > 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.
Thanks, Feng
> Hmm? > > -- > Regards/Gruss, > Boris. > > SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg
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