linux-kernel - Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local memory and limit readahead pages

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Date:	Mon, 10 Feb 2014 17:55:58 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	David Cohen <david.a.cohen@...ux.intel.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Damien Ramonda <damien.ramonda@...el.com>,
	Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V5] mm readahead: Fix readahead fail for no local
 memory and limit readahead pages

On 02/10/2014 03:35 PM, David Rientjes wrote:
> On Mon, 10 Feb 2014, Raghavendra K T wrote:
>
>> As you rightly pointed , I 'll drop remote memory term and use
>> something like  :
>>
>> "* Ensure readahead success on a memoryless node cpu. But we limit
>>   * the readahead to 4k pages to avoid trashing page cache." ..
>>
>
> I don't know how to proceed here after pointing it out twice, I'm afraid.
>
> numa_mem_id() is local memory for a memoryless node.  node_present_pages()
> has no place in your patch.

Hi David,  I am happy to see your pointer reg. numa_mem_id(). I did not
meant to be ignoring/offensive .. sorry if conversation thought to be so.

So I understood that you are suggesting implementations like below

1) I do not have problem with the below approach, I could post this in
next version.
( But this did not include 4k limit Linus mentioned to apply)

unsigned long max_sane_readahead(unsigned long nr)
{
         unsigned long local_free_page;
         int nid;

         nid = numa_mem_id();

         /*
          * We sanitize readahead size depending on free memory in
          * the local node.
          */
         local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
                           + node_page_state(nid, NR_FREE_PAGES);
         return min(nr, local_free_page / 2);
}

2) I did not go for below because Honza (Jan Kara) had some
concerns for 4k limit for normal case, and since I am not
the expert, I was waiting for opinions.

unsigned long max_sane_readahead(unsigned long nr)
{
         unsigned long local_free_page, sane_nr;
         int nid;

         nid = numa_mem_id();
	/* limit the max readahead to 4k pages */
	sane_nr = min(nr, MAX_REMOTE_READAHEAD);

         /*
          * We sanitize readahead size depending on free memory in
          * the local node.
          */
         local_free_page = node_page_state(nid, NR_INACTIVE_FILE)
                           + node_page_state(nid, NR_FREE_PAGES);
         return min(sane_nr, local_free_page / 2);
}

>
>> Regarding ACCESS_ONCE, since we will have to add
>> inside the function and still there is nothing that could prevent us
>> getting run on different cpu with a different node (as Andrew ponted), I have
>> not included in current patch that I am posting.
>> Moreover this case is hopefully not fatal since it is just a hint for
>> readahead we can do.
>>
>
> I have no idea why you think the ACCESS_ONCE() is a problem.  It's relying
> on gcc's implementation to ensure that the equation is done only for one
> node.  It has absolutely nothing to do with the fact that the process may
> be moved to another cpu upon returning or even immediately after the
> calculation is done.  Is it possible that node0 has 80% of memory free and
> node1 has 80% of memory inactive?  Well, then your equation doesn't work
> quite so well if the process moves.
>
> There is no downside whatsoever to using it, I have no idea why you think
> it's better without it.

I have no problem introducing ACESSS_ONCE too. But I skipped only
after I got the below error.

mm/readahead.c: In function ‘max_sane_readahead’:
mm/readahead.c:246: error: lvalue required as unary ‘&’ operand

>
>> So there are many possible implementation:
>> (1) use numa_mem_id(), apply freepage limit  and use 4k page limit for all
>> case
>> (Jan had reservation about this case)
>>
>> (2)for normal case:    use free memory calculation and do not apply 4k
>>      limit (no change).
>>     for memoryless cpu case:  use numa_mem_id for more accurate
>>      calculation of limit and also apply 4k limit.
>>
>> (3) for normal case:   use free memory calculation and do not apply 4k
>>      limit (no change).
>>      for memoryless case: apply 4k page limit
>>
>> (4) use numa_mem_id() and apply only free page limit..
>>
>> So, I ll be resending the patch with changelog and comment changes
>> based on your and Andrew's feedback (type (3) implementation).
>>
>
> It's frustrating to have to say something three times.  Ask yourself what
> happens if ALL NODES WITH CPUS DO NOT HAVE MEMORY?
>

True, this is the reason why we could go for implementation (1) I posted
above. It was just that I did not want to float a new version without
knowing whether Andrew was expecting new patch or change log updates.

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