linux-kernel - Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev

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Date:   Wed, 3 Nov 2021 22:54:09 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] mm/mremap_pages: Save a few cycles in 'get_dev_pagemap()'

Le 03/11/2021 à 22:41, Matthew Wilcox a écrit :
> On Wed, Nov 03, 2021 at 10:35:34PM +0100, Christophe JAILLET wrote:
>> Use 'percpu_ref_tryget_live_rcu()' instead of 'percpu_ref_tryget_live()' to
>> save a few cycles when it is known that the rcu lock is already
>> taken/released.
> 
> If this is really important, we can add an __xa_load() which doesn't
> take the RCU read lock.

There are a few:
    rcu_read_lock();
    mem = xa_load(...);
    rcu_read_unlock();
patterns here and there.

I don't have any numbers of if saving some rcu_read_lock/rcu_read_unlock 
would be useful in these cases.

The only numbers I have are in [1].

[1]: 
https://lore.kernel.org/linux-kernel/cover.1634822969.git.asml.silence@gmail.com/

CJ


> 
> I honestly think that the xarray is the wrong data structure here,
> and we'd be better off with a simple array of (start, pointer)
> tuples.
> 
>> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> ---
>>   mm/memremap.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memremap.c b/mm/memremap.c
>> index 84de22c14567..012e8d23d365 100644
>> --- a/mm/memremap.c
>> +++ b/mm/memremap.c
>> @@ -506,7 +506,7 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
>>   	/* fall back to slow path lookup */
>>   	rcu_read_lock();
>>   	pgmap = xa_load(&pgmap_array, PHYS_PFN(phys));
>> -	if (pgmap && !percpu_ref_tryget_live(pgmap->ref))
>> +	if (pgmap && !percpu_ref_tryget_live_rcu(pgmap->ref))
>>   		pgmap = NULL;
>>   	rcu_read_unlock();
>>   
>> -- 
>> 2.30.2
>>
>>
>