linux-kernel - Re: [PATCH RFC v3 06/35] mm: cma: Make CMA_ALLOC

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Message-ID: <ZbjkTFEvSvyHNqmu@raptor>
Date: Tue, 30 Jan 2024 11:58:04 +0000
From: Alexandru Elisei <alexandru.elisei@....com>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: catalin.marinas@....com, will@...nel.org, oliver.upton@...ux.dev,
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Subject: Re: [PATCH RFC v3 06/35] mm: cma: Make CMA_ALLOC_SUCCESS/FAIL count
 the number of pages

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?

> 
> > (allocated CMA pages - CMA pages allocated by drivers* - CMA pages
> > released) * 32. That is valuable information for software and hardware
> > designers.
> > 
> > Besides that, for every iteration of the series, this has proven invaluable
> > for discovering bugs with freeing and/or reserving tag storage pages.
> 
> I am afraid that might not be enough justification for getting something
> merged mainline.
> 
> > 
> > *that would require userspace reading cma_alloc_success and
> > cma_release_success before any tagged allocations are performed.
> 
> While assuming that no other non-memory-tagged CMA based allocation amd free
> call happens in the meantime ? That would be on real thin ice.
> 
> I suppose arm64 tagged memory specific allocation or free related counters
> need to be created on the caller side, including arch_free_pages_prepare().

I'll think about this. At the very least, I can add tracepoints.

Thanks,
Alex