linux-kernel - Re: [v5 09/15] sparc64: optimized struct page zeroing

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Message-ID: <4d98e852-15c0-1b66-a472-776ba1d51a6b@oracle.com>
Date:   Fri, 4 Aug 2017 09:50:57 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, kasan-dev@...glegroups.com, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net,
        willy@...radead.org, mhocko@...nel.org
Subject: Re: [v5 09/15] sparc64: optimized struct page zeroing

Hi Sam,

Thank you for looking at this. I will update patch description, and as 
you suggested replace memset() via static assert in next iteration.

Pasha

On 08/04/2017 01:37 AM, Sam Ravnborg wrote:
> Hi Pavel.
> 
> On Thu, Aug 03, 2017 at 05:23:47PM -0400, Pavel Tatashin wrote:
>> Add an optimized mm_zero_struct_page(), so struct page's are zeroed without
>> calling memset(). We do eight regular stores, thus avoid cost of membar.
> 
> The commit message does no longer reflect the implementation,
> and should be updated.
> 
>>
>> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
>> Reviewed-by: Steven Sistare <steven.sistare@...cle.com>
>> Reviewed-by: Daniel Jordan <daniel.m.jordan@...cle.com>
>> Reviewed-by: Bob Picco <bob.picco@...cle.com>
>> ---
>>   arch/sparc/include/asm/pgtable_64.h | 32 ++++++++++++++++++++++++++++++++
>>   1 file changed, 32 insertions(+)
>>
>> diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
>> index 6fbd931f0570..be47537e84c5 100644
>> --- a/arch/sparc/include/asm/pgtable_64.h
>> +++ b/arch/sparc/include/asm/pgtable_64.h
>> @@ -230,6 +230,38 @@ extern unsigned long _PAGE_ALL_SZ_BITS;
>>   extern struct page *mem_map_zero;
>>   #define ZERO_PAGE(vaddr)	(mem_map_zero)
>>   
>> +/* This macro must be updated when the size of struct page grows above 80
>> + * or reduces below 64.
>> + * The idea that compiler optimizes out switch() statement, and only
>> + * leaves clrx instructions or memset() call.
>> + */
>> +#define	mm_zero_struct_page(pp) do {					\
>> +	unsigned long *_pp = (void *)(pp);				\
>> +									\
>> +	/* Check that struct page is 8-byte aligned */			\
>> +	BUILD_BUG_ON(sizeof(struct page) & 7);				\
> Would also be good to catch if sizeof > 80 so we do not silently
> migrate to the suboptimal version (silent at build time).
> Can you at build time catch if size is no any of: 64, 72, 80
> and simplify the below a little?
> 
> 	Sam
>