linux-kernel - Re: [PATCH v2 22/25] powerpc32: move xxxxx_dcache

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Message-ID: <1442951752.29498.58.camel@transmode.se>
Date:	Tue, 22 Sep 2015 19:55:53 +0000
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	"scottwood@...escale.com" <scottwood@...escale.com>
CC:	"christophe.leroy@....fr" <christophe.leroy@....fr>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"mpe@...erman.id.au" <mpe@...erman.id.au>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 22/25] powerpc32: move xxxxx_dcache_range() functions
 inline

On Tue, 2015-09-22 at 14:42 -0500, Scott Wood wrote:
> On Tue, 2015-09-22 at 19:34 +0000, Joakim Tjernlund wrote:
> > On Tue, 2015-09-22 at 13:58 -0500, Scott Wood wrote:
> > > On Tue, 2015-09-22 at 18:12 +0000, Joakim Tjernlund wrote:
> > > > On Tue, 2015-09-22 at 18:51 +0200, Christophe Leroy wrote:
> > > > > flush/clean/invalidate _dcache_range() functions are all very
> > > > > similar and are quite short. They are mainly used in __dma_sync()
> > > > > perf_event locate them in the top 3 consumming functions during
> > > > > heavy ethernet activity
> > > > > 
> > > > > They are good candidate for inlining, as __dma_sync() does
> > > > > almost nothing but calling them
> > > > > 
> > > > > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > > > > ---
> > > > > New in v2
> > > > > 
> > > > >  arch/powerpc/include/asm/cacheflush.h | 55 
> > > > > +++++++++++++++++++++++++++--
> > > > >  arch/powerpc/kernel/misc_32.S         | 65 --------------------------
> > > > > ----
> > > > > -----
> > > > >  arch/powerpc/kernel/ppc_ksyms.c       |  2 ++
> > > > >  3 files changed, 54 insertions(+), 68 deletions(-)
> > > > > 
> > > > > diff --git a/arch/powerpc/include/asm/cacheflush.h 
> > > > > b/arch/powerpc/include/asm/cacheflush.h
> > > > > index 6229e6b..6169604 100644
> > > > > --- a/arch/powerpc/include/asm/cacheflush.h
> > > > > +++ b/arch/powerpc/include/asm/cacheflush.h
> > > > > @@ -47,12 +47,61 @@ static inline void 
> > > > > __flush_dcache_icache_phys(unsigned long physaddr)
> > > > >  }
> > > > >  #endif
> > > > >  
> > > > > -extern void flush_dcache_range(unsigned long start, unsigned long 
> > > > > stop);
> > > > >  #ifdef CONFIG_PPC32
> > > > > -extern void clean_dcache_range(unsigned long start, unsigned long 
> > > > > stop);
> > > > > -extern void invalidate_dcache_range(unsigned long start, unsigned 
> > > > > long 
> > > > > stop);
> > > > > +/*
> > > > > + * Write any modified data cache blocks out to memory and invalidate 
> > > > > them.
> > > > > + * Does not invalidate the corresponding instruction cache blocks.
> > > > > + */
> > > > > +static inline void flush_dcache_range(unsigned long start, unsigned 
> > > > > long 
> > > > > stop)
> > > > > +{
> > > > > +   void *addr = (void *)(start & ~(L1_CACHE_BYTES - 1));
> > > > > +   unsigned int size = stop - (unsigned long)addr + (L1_CACHE_BYTES -
> > > > > 1);
> > > > > +   unsigned int i;
> > > > > +
> > > > > +   for (i = 0; i < size >> L1_CACHE_SHIFT; i++, addr += 
> > > > > L1_CACHE_BYTES)
> > > > > +           dcbf(addr);
> > > > > +   if (i)
> > > > > +           mb();   /* sync */
> > > > > +}
> > > > 
> > > > This feels optimized for the uncommon case when there is no 
> > > > invalidation.
> > > 
> > > If you mean the "if (i)", yes, that looks odd.
> > 
> > Yes.
> > 
> > > 
> > > > I THINK it would be better to bail early 
> > > 
> > > Bail under what conditions?
> > 
> > test for "i = 0" and return. 
> 
> Why bother?

I usally find it better to dela with special cases upfront så the rest doesn't need to
bother. i=0 is a NOP and it is clearer to show that upfront.

> 
> > 
> > > 
> > > > and use do { .. } while(--i); instead.
> > > 
> > > GCC knows how to optimize loops.  Please don't make them less readable.
> > 
> > Been a while since I checked but it used to be bad att transforming post 
> > inc to pre inc/dec
> > I remain unconvinced until I have seen it.
> 
> I would expect it to use bdnz for this loop, as the loop variable isn't 
> referenced in the loop body.
> 
> And generally the one proposing uglification-for-optimization should provide 
> the evidence. :-)

When it comes to gcc, past history is my evidence until proven otherwise :)
Maybe I will check again ...--
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