linux-kernel - Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment

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Date:   Wed, 29 Sep 2021 16:40:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ping Fang <pifang@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment >
 1

On 29.09.21 16:30, Uladzislau Rezki wrote:
>>
>> So the idea is that once we run into a dead end because we took a left
>> subtree, we rollback to the next possible rigth subtree and try again.
>> If we run into another dead end, we repeat ... thus, this can now happen
>> more than once.
>>
>> I assume the only implication is that this can now be slower in some
>> corner cases with larger alignment, because it might take longer to find
>> something suitable. Fair enough.
>>
> Yep, your understanding is correct regarding the tree traversal. If no
> suitable block
> is found in left sub-tree we roll-back and check right one. So it can
> be(the scanning)
> more than one time.
> 
> I did some performance analyzing using vmalloc test suite to figure
> out a performance
> loss for allocations with specific alignment. On that syntactic test i
> see approx. 30%
> of degradation:

How realistic is that test case? I assume most alignment we're dealing 
with is:
* 1/PAGE_SIZE
* huge page size (for automatic huge page placing)

> 
> 2.225 microseconds vs 1.496 microseconds. That time includes both
> vmalloc() and vfree()
> calls. I do not consider it as a big degrade, but from the other hand
> we can still adjust the
> search length for alignments > one page:
> 
> # add it on top of previous proposal and search length instead of size
> length = align > PAGE_SIZE ? size + align:size;

That will not allow to place huge pages in the case of kasan. And I 
consider that more important than optimizing a syntactic test :) My 2 cents.

-- 
Thanks,

David / dhildenb