From: David Hildenbrand <david@redhat.com>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Ping Fang <pifang@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@suse.com>,
Oscar Salvador <osalvador@suse.de>,
Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment > 1
Date: Wed, 29 Sep 2021 17:05:08 +0200 [thread overview]
Message-ID: <689b7c24-623d-c01e-6c0f-ad430f1fa3ae@redhat.com> (raw)
In-Reply-To: <CA+KHdyWFjtoVqGd=7-yp33G-5WcZCtf80BaAk+3jx2bW5FCfWA@mail.gmail.com>
On 29.09.21 16:49, Uladzislau Rezki wrote:
> On Wed, Sep 29, 2021 at 4:40 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> 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)
>>
> Well that is synthetic test. Most of the alignments are 1 or PAGE_SIZE.
> There are users which use internal API where you can specify an alignment
> you want but those are mainly like KASAN, module alloc, etc.
>
>>>
>>> 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.
>>
> Could you please to be more specific? I mean how is it connected with huge
> pages mappings? Huge-pages are which have order > 0. Or you mean that
> a special alignments are needed for mapping huge pages?
Let me try to clarify:
KASAN does an exact allocation when onlining a memory block,
__vmalloc_node_range() will try placing huge pages first, increasing the
alignment to e.g., "1 << PMD_SHIFT".
If we increase the search length in find_vmap_lowest_match(), that
search will fail if the exact allocation is surrounded by other
allocations. In that case, we won't place a huge page although we could
-- because find_vmap_lowest_match() would be imprecise for alignments >
PAGE_SIZE.
Memory blocks we online/offline on x86 are at least 128MB. The KASAN
"overhead" we have to allocate is 1/8 of that -- 16 MB, so essentially 8
huge pages.
__vmalloc_node_range() will increase the alignment to 2MB to try placing
huge pages first. find_vmap_lowest_match() will search within the given
exact 16MB are a 18MB area (size + align), which won't work. So
__vmalloc_node_range() will fallback to the original PAGE_SIZE alignment
and shift=PAGE_SHIFT.
__vmalloc_area_node() will set the set_vm_area_page_order effectively to
0 -- small pages.
Does that make sense or am I missing something?
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2021-09-29 15:05 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-08 13:27 [PATCH v1] mm/vmalloc: fix exact allocations with an alignment > 1 David Hildenbrand
2021-09-13 8:44 ` Uladzislau Rezki
2021-09-14 19:15 ` Uladzislau Rezki
2021-09-15 9:02 ` David Hildenbrand
2021-09-16 19:34 ` Uladzislau Rezki
2021-09-17 8:47 ` David Hildenbrand
2021-09-21 22:13 ` Uladzislau Rezki
2021-09-22 8:34 ` David Hildenbrand
2021-09-22 10:41 ` Uladzislau Rezki
2021-09-23 17:42 ` David Hildenbrand
2021-09-24 12:42 ` David Hildenbrand
2021-09-29 14:30 ` Uladzislau Rezki
2021-09-29 14:40 ` David Hildenbrand
2021-09-29 14:49 ` Uladzislau Rezki
2021-09-29 15:05 ` David Hildenbrand [this message]
2021-09-29 16:08 ` Uladzislau Rezki
2021-09-29 16:10 ` David Hildenbrand
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