linux-kernel - Re: [PATCH] powerpc/32: Don't use lmw/stmw for saving/restoring non volatile regs

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Date:   Tue, 24 Aug 2021 07:54:22 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/32: Don't use lmw/stmw for saving/restoring non
 volatile regs



Le 23/08/2021 à 20:46, Segher Boessenkool a écrit :
> On Mon, Aug 23, 2021 at 03:29:12PM +0000, Christophe Leroy wrote:
>> Instructions lmw/stmw are interesting for functions that are rarely
>> used and not in the cache, because only one instruction is to be
>> copied into the instruction cache instead of 19. However those
>> instruction are less performant than 19x raw lwz/stw as they require
>> synchronisation plus one additional cycle.
> 
> lmw takes N+2 cycles for loading N words on 603/604/750/7400, and N+3 on
> 7450.  stmw takes N+1 cycles for storing N words on 603, N+2 on 604/750/
> 7400, and N+3 on 7450 (load latency is 3 instead of 2 on 7450).
> 
> There is no synchronisation needed, although there is some serialisation,
> which of course doesn't mean much since there can be only 6 or 8 or so
> insns executing at once anyway.

Yes I meant serialisation, isn't it the same as synchronisation ?

> 
> So, these insns are almost never slower, they can easily win cycles back
> because of the smaller code, too.
> 
> What 32-bit core do you see where load/store multiple are more than a
> fraction of a cycle (per memory access) slower?
> 
>> SAVE_NVGPRS / REST_NVGPRS are used in only a few places which are
>> mostly in interrupts entries/exits and in task switch so they are
>> likely already in the cache.
> 
> Nothing is likely in the cache on the older cores (except in
> microbenchmarks), the caches are not big enough for that!

Even syscall entries/exit pathes and/or most frequent interrupts entries and interrupt exit ?


> 
>> Using standard lwz improves null_syscall selftest by:
>> - 10 cycles on mpc832x.
>> - 2 cycles on mpc8xx.
> 
> And in real benchmarks?

Don't know, what benchmark should I use to evaluate syscall entry/exit if 'null_syscall' selftest is 
not relevant ?

> 
> On mpccore both lmw and stmw are only N+1 btw.  But the serialization
> might cost another cycle here?
> 

That coherent on MPC8xx, that's only 2 cycles.
But on the mpc832x which has a e300c2 core, it looks like I have 10 cycles difference. Is anything 
wrong ?

Christophe