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Date: Thu, 20 Oct 2022 15:19:42 +0800
From: Gavin Shan <gshan@...hat.com>
To: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc: kvm@...r.kernel.org, maz@...nel.org, linux-kernel@...r.kernel.org,
zhenyzha@...hat.com, shan.gavin@...il.com, kvmarm@...ts.linux.dev,
pbonzini@...hat.com, shuah@...nel.org,
kvmarm@...ts.cs.columbia.edu, ajones@...tanamicro.com
Subject: Re: [PATCH 4/6] KVM: selftests: memslot_perf_test: Support variable
guest page size
On 10/20/22 4:18 AM, Maciej S. Szmigiero wrote:
> On 19.10.2022 02:26, Gavin Shan wrote:
>> On 10/18/22 11:56 PM, Maciej S. Szmigiero wrote:
>>> On 18.10.2022 02:51, Gavin Shan wrote:
>>>> On 10/18/22 8:46 AM, Gavin Shan wrote:
>>>>> On 10/18/22 5:31 AM, Maciej S. Szmigiero wrote:
>>>>>> On 14.10.2022 09:19, Gavin Shan wrote:
>>>>>>> The test case is obviously broken on aarch64 because non-4KB guest
>>>>>>> page size is supported. The guest page size on aarch64 could be 4KB,
>>>>>>> 16KB or 64KB.
>>>>>>>
>>>>>>> This supports variable guest page size, mostly for aarch64.
>>>>>>>
>>>>>>> - The host determines the guest page size when virtual machine is
>>>>>>> created. The value is also passed to guest through the synchronization
>>>>>>> area.
>>>>>>>
>>>>>>> - The number of guest pages are unknown until the virtual machine
>>>>>>> is to be created. So all the related macros are dropped. Instead,
>>>>>>> their values are dynamically calculated based on the guest page
>>>>>>> size.
>>>>>>>
>>>>>>> - The static checks on memory sizes and pages becomes dependent
>>>>>>> on guest page size, which is unknown until the virtual machine
>>>>>>> is about to be created. So all the static checks are converted
>>>>>>> to dynamic checks, done in check_memory_sizes().
>>>>>>>
>>>>>>> - As the address passed to madvise() should be aligned to host page,
>>>>>>> the size of page chunk is automatically selected, other than one
>>>>>>> page.
>>>>>>>
>>>>>>> - All other changes included in this patch are almost mechanical
>>>>>>> replacing '4096' with 'guest_page_size'.
>>>>>>>
>>>>>>> Signed-off-by: Gavin Shan <gshan@...hat.com>
>>>>>>> ---
>>>>>>> .../testing/selftests/kvm/memslot_perf_test.c | 191 +++++++++++-------
>>>>>>> 1 file changed, 115 insertions(+), 76 deletions(-)
>>>>>>>
>>>>>>> diff --git a/tools/testing/selftests/kvm/memslot_perf_test.c b/tools/testing/selftests/kvm/memslot_perf_test.c
>>>>>>> index d5aa9148f96f..d587bd952ff9 100644
>>>>>>> --- a/tools/testing/selftests/kvm/memslot_perf_test.c
>>>>>>> +++ b/tools/testing/selftests/kvm/memslot_perf_test.c
>>> (...)
>>>>>>> @@ -77,8 +61,7 @@ static_assert(MEM_TEST_UNMAP_SIZE_PAGES %
>>>>>>> * for the total size of 25 pages.
>>>>>>> * Hence, the maximum size here is 50 pages.
>>>>>>> */
>>>>>>> -#define MEM_TEST_MOVE_SIZE_PAGES (50)
>>>>>>> -#define MEM_TEST_MOVE_SIZE (MEM_TEST_MOVE_SIZE_PAGES * 4096)
>>>>>>> +#define MEM_TEST_MOVE_SIZE 0x32000
>>>>>>
>>>>>> The above number seems less readable than an explicit value of 50 pages.
>>>>>>
>>>>>> In addition to that, it's 50 pages only with 4k page size, so at least
>>>>>> the comment above needs to be updated to reflect this fact.
>>>>>>
>>>>>
>>>>> Yeah, I will change the comments like below in next revision.
>>>>>
>>>>> /*
>>>>> * When running this test with 32k memslots, actually 32763 excluding
>>>>> * the reserved memory slot 0, the memory for each slot is 0x4000 bytes.
>>>>> * The last slot contains 0x19000 bytes memory. Hence, the maximum size
>>>>> * here is 0x32000 bytes.
>>>>> */
>>>>>
>>>>
>>>> I will replace those numbers with readable ones like below :)
>>>>
>>>> /*
>>>> * When running this test with 32k memslots, actually 32763 excluding
>>>> * the reserved memory slot 0, the memory for each slot is 16KB. The
>>>> * last slot contains 100KB memory with the remaining 84KB. Hence,
>>>> * the maximum size is double of that (200KB)
>>>> */
>>>
>>> Still, these numbers are for x86, which has KVM_INTERNAL_MEM_SLOTS
>>> defined as 3.
>>>
>>> As far as I can see aarch64 has KVM_INTERNAL_MEM_SLOTS equal to 0, so
>>> this arch has 32766 slot available for the test memory.
>>>
>>> Quick calculations show that this will result in 112 KiB of memory in
>>> the last slot for 4 KiB page size (while for 64 KiB page size the
>>> maximum slot count for this test is 8192 anyway - not counting slot 0).
>>>
>>
>> It seems your calculation had (512MB+64KB), instead of (512MB+4KB).
>> In this particular patch, we still have (512MB+4KB). How about to change
>> like below in this patch. In next patch, it's adjusted accordingly after
>> we have (512MB+64KB).
>
> My review comment above referred to the final MEM_SIZE value after the
> whole series, so 512 MiB + 64 KiB.
>
> I placed that review comment on patch 4 since it's the only patch in this
> series that modified the code comment about MEM_TEST_MOVE_SIZE.
>
>>
>> (1) In this patch, the comment is changed to as below
>>
>> /*
>> * We have different number of memory slots, excluding the reserved
>> * memory slot 0, on various architectures and configurations. The
>> * memory size in this test is calculated by doubling the maximal
>> * memory size in last memory slot, with alignment to the largest
>> * supported page size (64KB).
>> *
>> * architecture slots memory-per-slot memory-on-last-slot
>> * --------------------------------------------------------------
>> * x86-4KB 32763 16KB 100KB
>> * arm64-4KB 32766 16KB 52KB
>> * arm64-64KB 8192 64KB 64KB
>> */
>> #define MEM_TEST_MOVE_SIZE 0x40000 /* 256KB */
>>
>> (2) In the next patch, where we have (512MB+64KB) after the various
>> memory sizes are consolidated, It is adjusted accordingly as below.
>>
>> /*
>> * We have different number of memory slots, excluding the reserved
>> * memory slot 0, on various architectures and configurations. The
>> * memory size in this test is calculated by doubling the maximal
>> * memory size in last memory slot, with alignment to the largest
>> * supported page size (64KB).
>> *
>> * architecture slots memory-per-slot memory-on-last-slot
>> * --------------------------------------------------------------
>> * x86-4KB 32763 16KB 160KB
>> * arm64-4KB 32766 16KB 112KB
>> * arm64-64KB 8192 64KB 128KB
>> */
>> #define MEM_TEST_MOVE_SIZE 0x50000 /* 320KB */
>
> Now MEM_TEST_MOVE_SIZE is too high for arm64-4KB and arm64-64KB cases
> (it needs 160 KiB in the last slot but has less available in these two
> cases).
>
> Using a test size of 192 KiB instead seems like a small difference
> from the original size of 200 KiB, while still being aligned to
> 64 KiB.
>
> The move benchmarks runtime difference on x86-4KB with this size
> (compared to sizes of 200 KiB and 320 KiB) seems to be negligible.
>
> Since it's an odd number of 64 KiB pages (3) the code that halves
> this number of pages will need to be adjusted to operate on raw
> sizes instead.
>
> I can see a single block of code that will need such adjustment:
>> if (lastpages < move_pages / 2) {
>> *maxslots = 0;
>> return false;
>> }
>
> Similar remark goes for the case (1) above, where you'll probably need
> to use 64 KiB test area size (it's only an intermediate form of code
> before the final patch changes this value so it's fine if it doesn't
> perform as well as the final form of the code).
>
Maciej, all your comments make sense to me. It really took me some times
to do the calculation. I just posted v3 to address all your comments.
Hopefully, there is nothing missed. Please go ahead to review v3 directly
when you get a chance.
v3: https://lore.kernel.org/kvmarm/20221020071209.559062-1-gshan@redhat.com/T/#t
In v3, the comments about MEM_TEST_MOVE_SIZE is fixed in PATCH[v3 4/6],
but it's 64KB. In PATCH[v3 5/6], it's fixed up to 192KB and memory size
is used for the comparison in test_memslot_move_prepare().
Thanks,
Gavin
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