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Message-ID: <8cd39b48-7fb8-40b2-8d6c-e6fc2b48f86d@arm.com>
Date: Wed, 31 Jan 2024 10:10:05 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Alexandru Elisei <alexandru.elisei@....com>
Cc: catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev,
maz@...nel.org, james.morse@....com, suzuki.poulose@....com,
yuzenghui@...wei.com, arnd@...db.de, akpm@...ux-foundation.org,
mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
vincent.guittot@...aro.org, dietmar.eggemann@....com, rostedt@...dmis.org,
bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
vschneid@...hat.com, mhiramat@...nel.org, rppt@...nel.org, hughd@...gle.com,
pcc@...gle.com, steven.price@....com, vincenzo.frascino@....com,
david@...hat.com, eugenis@...gle.com, kcc@...gle.com, hyesoo.yu@...sung.com,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
linux-arch@...r.kernel.org, linux-mm@...ck.org,
linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v3 06/35] mm: cma: Make CMA_ALLOC_SUCCESS/FAIL count
the number of pages
On 1/30/24 17:28, Alexandru Elisei wrote:
> Hi,
>
> On Tue, Jan 30, 2024 at 10:22:11AM +0530, Anshuman Khandual wrote:
>>
>> On 1/29/24 17:21, Alexandru Elisei wrote:
>>> Hi,
>>>
>>> On Mon, Jan 29, 2024 at 02:54:20PM +0530, Anshuman Khandual wrote:
>>>>
>>>> On 1/25/24 22:12, Alexandru Elisei wrote:
>>>>> The CMA_ALLOC_SUCCESS, respectively CMA_ALLOC_FAIL, are increased by one
>>>>> after each cma_alloc() function call. This is done even though cma_alloc()
>>>>> can allocate an arbitrary number of CMA pages. When looking at
>>>>> /proc/vmstat, the number of successful (or failed) cma_alloc() calls
>>>>> doesn't tell much with regards to how many CMA pages were allocated via
>>>>> cma_alloc() versus via the page allocator (regular allocation request or
>>>>> PCP lists refill).
>>>>>
>>>>> This can also be rather confusing to a user who isn't familiar with the
>>>>> code, since the unit of measurement for nr_free_cma is the number of pages,
>>>>> but cma_alloc_success and cma_alloc_fail count the number of cma_alloc()
>>>>> function calls.
>>>>>
>>>>> Let's make this consistent, and arguably more useful, by having
>>>>> CMA_ALLOC_SUCCESS count the number of successfully allocated CMA pages, and
>>>>> CMA_ALLOC_FAIL count the number of pages the cma_alloc() failed to
>>>>> allocate.
>>>>>
>>>>> For users that wish to track the number of cma_alloc() calls, there are
>>>>> tracepoints for that already implemented.
>>>>>
>>>>> Signed-off-by: Alexandru Elisei <alexandru.elisei@....com>
>>>>> ---
>>>>> mm/cma.c | 4 ++--
>>>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/cma.c b/mm/cma.c
>>>>> index f49c95f8ee37..dbf7fe8cb1bd 100644
>>>>> --- a/mm/cma.c
>>>>> +++ b/mm/cma.c
>>>>> @@ -517,10 +517,10 @@ struct page *cma_alloc(struct cma *cma, unsigned long count,
>>>>> pr_debug("%s(): returned %p\n", __func__, page);
>>>>> out:
>>>>> if (page) {
>>>>> - count_vm_event(CMA_ALLOC_SUCCESS);
>>>>> + count_vm_events(CMA_ALLOC_SUCCESS, count);
>>>>> cma_sysfs_account_success_pages(cma, count);
>>>>> } else {
>>>>> - count_vm_event(CMA_ALLOC_FAIL);
>>>>> + count_vm_events(CMA_ALLOC_FAIL, count);
>>>>> if (cma)
>>>>> cma_sysfs_account_fail_pages(cma, count);
>>>>> }
>>>> Without getting into the merits of this patch - which is actually trying to do
>>>> semantics change to /proc/vmstat, wondering how is this even related to this
>>>> particular series ? If required this could be debated on it's on separately.
>>> Having the number of CMA pages allocated and the number of CMA pages freed
>>> allows someone to infer how many tagged pages are in use at a given time:
>> That should not be done in CMA which is a generic multi purpose allocator.
> Ah, ok. Let me rephrase that: Having the number of CMA pages allocated, the
> number of failed CMA page allocations and the number of freed CMA pages
> allows someone to infer how many CMA pages are in use at a given time.
> That's valuable information for software designers and system
> administrators, as it allows them to tune the number of CMA pages available
> in a system.
>
> Or put another way: what would you consider to be more useful? Knowing the
> number of cma_alloc()/cma_release() calls, or knowing the number of pages
> that cma_alloc()/cma_release() allocated or freed?
There is still value in knowing how many times cma_alloc() succeeded or failed
regardless of the cumulative number pages involved over the time. Actually the
count helps to understand how cma_alloc() performed overall as an allocator.
But on the cma_release() path there is no chances of failure apart from - just
when the caller itself provides an wrong input. So there are no corresponding
CMA_RELEASE_SUCCESS/CMA_RELEASE_FAIL vmstat counters in there - for a reason !
Coming back to CMA based pages being allocated and freed, there is already an
interface via sysfs (CONFIG_CMA_SYSFS) which gets updated in cma_alloc() path
via cma_sysfs_account_success_pages() and cma_sysfs_account_fail_pages().
#ls /sys/kernel/mm/cma/<name>
alloc_pages_fail alloc_pages_success
Why these counters could not meet your requirements ? Also 'struct cma' can
be updated to add an element 'nr_pages_freed' to be tracked in cma_release(),
providing free pages count as well.
There are additional debug fs based elements (CONFIG_CMA_DEBUGFS) available.
#ls /sys/kernel/debug/cma/<name>
alloc base_pfn bitmap count free maxchunk order_per_bit used
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