Re: [PATCH v10 3/6] mm: Introduce Reported pages

From: David Hildenbrand <david@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
	Dave Hansen <dave.hansen@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	Michal Hocko <mhocko@kernel.org>, linux-mm <linux-mm@kvack.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	linux-arm-kernel@lists.infradead.org,
	Oscar Salvador <osalvador@suse.de>,
	Yang Zhang <yang.zhang.wz@gmail.com>,
	Pankaj Gupta <pagupta@redhat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Nitesh Narayan Lal <nitesh@redhat.com>,
	Rik van Riel <riel@surriel.com>,
	lcapitulino@redhat.com, "Wang, Wei W" <wei.w.wang@intel.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>
Subject: Re: [PATCH v10 3/6] mm: Introduce Reported pages
Date: Mon, 23 Sep 2019 17:53:46 +0200	[thread overview]
Message-ID: <c6a9e5ae-50e4-379c-164e-eb9da0cb2868@redhat.com> (raw)
In-Reply-To: <20190923114946-mutt-send-email-mst@kernel.org>

On 23.09.19 17:50, Michael S. Tsirkin wrote:
> On Mon, Sep 23, 2019 at 05:47:24PM +0200, David Hildenbrand wrote:
>> On 23.09.19 17:45, David Hildenbrand wrote:
>>> On 23.09.19 17:37, Michael S. Tsirkin wrote:
>>>> On Mon, Sep 23, 2019 at 08:28:00AM -0700, Alexander Duyck wrote:
>>>>> On Mon, Sep 23, 2019 at 8:00 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>>>
>>>>>> On Mon, Sep 23, 2019 at 07:50:15AM -0700, Alexander Duyck wrote:
>>>>>>>>> +static inline void
>>>>>>>>> +page_reporting_reset_boundary(struct zone *zone, unsigned int order, int mt)
>>>>>>>>> +{
>>>>>>>>> +     int index;
>>>>>>>>> +
>>>>>>>>> +     if (order < PAGE_REPORTING_MIN_ORDER)
>>>>>>>>> +             return;
>>>>>>>>> +     if (!test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
>>>>>>>>> +             return;
>>>>>>>>> +
>>>>>>>>> +     index = get_reporting_index(order, mt);
>>>>>>>>> +     reported_boundary[index] = &zone->free_area[order].free_list[mt];
>>>>>>>>> +}
>>>>>>>>
>>>>>>>> So this seems to be costly.
>>>>>>>> I'm guessing it's the access to flags:
>>>>>>>>
>>>>>>>>
>>>>>>>>         /* zone flags, see below */
>>>>>>>>         unsigned long           flags;
>>>>>>>>
>>>>>>>>         /* Primarily protects free_area */
>>>>>>>>         spinlock_t              lock;
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> which is in the same cache line as the lock.
>>>>>>>
>>>>>>> I'm not sure what you mean by this being costly?
>>>>>>
>>>>>> I've just been wondering why does will it scale report a 1.5% regression
>>>>>> with this patch.
>>>>>
>>>>> Are you talking about data you have collected from a test you have
>>>>> run, or the data I have run?
>>>>
>>>> About the kernel test robot auto report that was sent recently.
>>>
>>> https://lkml.org/lkml/2019/9/21/112
>>>
>>> And if I'm correct, that regression is observable in case reporting is
>>> not enabled. (so with this patch applied only, e.g., on a bare-metal system)
>>>
>>
>> To be even more precise: # CONFIG_PAGE_REPORTING is not set
> 
> Even if it was, I'd hope for 0 overhead when not present runtime.
> 

Right, because it will be included mostly in all kernels that support
virtio-balloon, so it applies to most distributions.

-- 

Thanks,

David / dhildenb