linux-kernel - Re: [PATCH v2 00/35] PREEMPT_AUTO: support lazy rescheduling

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Date: Thu, 27 Jun 2024 15:56:18 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: Ankur Arora <ankur.a.arora@...cle.com>, Shrikanth Hegde
 <sshegde@...ux.ibm.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, tglx@...utronix.de,
 peterz@...radead.org, torvalds@...ux-foundation.org, paulmck@...nel.org,
 rostedt@...dmis.org, mark.rutland@....com, juri.lelli@...hat.com,
 joel@...lfernandes.org, raghavendra.kt@....com,
 boris.ostrovsky@...cle.com, konrad.wilk@...cle.com, LKML
 <linux-kernel@...r.kernel.org>, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2 00/35] PREEMPT_AUTO: support lazy rescheduling

Ankur Arora <ankur.a.arora@...cle.com> writes:
> Shrikanth Hegde <sshegde@...ux.ibm.com> writes:
>> ...
>> This was the patch which I tried to make it per cpu for powerpc: It boots and runs workload.
>> Implemented a simpler one instead of folding need resched into preempt count. By hacky way avoided
>> tif_need_resched calls as didnt affect the throughput. Hence kept it simple. Below is the patch
>> for reference. It didn't help fix the regression unless I implemented it wrongly.
>>
>> diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
>> index 1d58da946739..374642288061 100644
>> --- a/arch/powerpc/include/asm/paca.h
>> +++ b/arch/powerpc/include/asm/paca.h
>> @@ -268,6 +268,7 @@ struct paca_struct {
>>  	u16 slb_save_cache_ptr;
>>  #endif
>>  #endif /* CONFIG_PPC_BOOK3S_64 */
>> +	int preempt_count;
>
> I don't know powerpc at all. But, would this cacheline be hotter
> than current_thread_info()::preempt_count?
>
>>  #ifdef CONFIG_STACKPROTECTOR
>>  	unsigned long canary;
>>  #endif

Assuming stack protector is enabled (it is in defconfig), that cache
line should quite be hot, because the canary is loaded as part of the
epilogue of many functions.

Putting preempt_count in the paca also means it's a single load/store to
access the value, just paca (in r13) + static offset. With the
preempt_count in thread_info it's two loads, one to load current from
the paca and then another to get the preempt_count.

It could be worthwhile to move preempt_count into the paca, but I'm not
convinced preempt_count is accessed enough for it to be a major
performance issue.

cheers