linux-kernel - Re: [PATCHv6 15/15] mm/vmstat: Add counter for memory accepting

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Date:   Wed, 1 Jun 2022 11:05:40 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Mike Rapoport <rppt@...nel.org>, marcelo.cerri@...onical.com,
        tim.gardner@...onical.com, khalid.elmously@...onical.com,
        philip.cox@...onical.com, x86@...nel.org, linux-mm@...ck.org,
        linux-coco@...ts.linux.dev, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv6 15/15] mm/vmstat: Add counter for memory accepting

On 17.05.22 17:34, Kirill A. Shutemov wrote:
> The counter increased every time kernel accepts a memory region.
> 
> The counter allows to see if memory acceptation is still ongoing and
> contributes to memory allocation latency.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/mm/unaccepted_memory.c | 1 +
>  include/linux/vm_event_item.h   | 3 +++
>  mm/vmstat.c                     | 3 +++
>  3 files changed, 7 insertions(+)
> 
> diff --git a/arch/x86/mm/unaccepted_memory.c b/arch/x86/mm/unaccepted_memory.c
> index 6ecd79101922..fe1dabfae326 100644
> --- a/arch/x86/mm/unaccepted_memory.c
> +++ b/arch/x86/mm/unaccepted_memory.c
> @@ -74,6 +74,7 @@ void accept_memory(phys_addr_t start, phys_addr_t end)
>  		}
>  
>  		bitmap_clear(bitmap, range_start, len);
> +		count_vm_events(ACCEPT_MEMORY, len * PMD_SIZE / PAGE_SIZE);
>  

It's a bit weird that this is accounted from arch code. Also, I'm a bit
confused about the granularity here (PMD_SIZE).

>  		/* In early boot nr_unaccepted is not yet initialized */
>  		if (nr_unaccepted) {
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 16a0a4fd000b..6a468164a2f9 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -136,6 +136,9 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>  #ifdef CONFIG_X86
>  		DIRECT_MAP_LEVEL2_SPLIT,
>  		DIRECT_MAP_LEVEL3_SPLIT,
> +#endif
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +		ACCEPT_MEMORY,
>  #endif
>  		NR_VM_EVENT_ITEMS
>  };
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index b75b1a64b54c..4c9197f32406 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1397,6 +1397,9 @@ const char * const vmstat_text[] = {
>  	"direct_map_level2_splits",
>  	"direct_map_level3_splits",
>  #endif
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +	"accept_memory",
> +#endif
>  #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
>  };
>  #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA || CONFIG_MEMCG */

How exactly would I be able to figure out if "memory acceptation is
still ongoing" if there is one last remaining page stuck at the tail of
the freelist?

Wouldn't it make more sense to actually count the number of unaccepted
pages in the buddy? Once that number drops to 0, one knows that there is
no unaccepted memory left in the buddy.

-- 
Thanks,

David / dhildenb