netdev - Re: [PATCH] iavf: use kvzalloc instead of kzalloc for rx/tx

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Message-ID: <cadd738c-b7a3-fd68-4883-2f23a07fb0ae@gmail.com>
Date:   Thu, 27 Aug 2020 02:55:28 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Li,Rongqing" <lirongqing@...du.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [PATCH] iavf: use kvzalloc instead of kzalloc for rx/tx_bi buffer



On 8/27/20 1:53 AM, Li,Rongqing wrote:
> 
> 
>> -----Original Message-----
>> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
>> Sent: Thursday, August 27, 2020 4:26 PM
>> To: Li,Rongqing <lirongqing@...du.com>; netdev@...r.kernel.org;
>> intel-wired-lan@...ts.osuosl.org
>> Subject: Re: [PATCH] iavf: use kvzalloc instead of kzalloc for rx/tx_bi buffer
>>
>>
>>
>> On 8/27/20 12:53 AM, Li RongQing wrote:
>>> when changes the rx/tx ring to 4096, kzalloc may fail due to a
>>> temporary shortage on slab entries.
>>>
>>> kvmalloc is used to allocate this memory as there is no need to have
>>> this memory area physical continuously.
>>>
>>> Signed-off-by: Li RongQing <lirongqing@...du.com>
>>> ---
>>
>>
>> Well, fallback to vmalloc() overhead because order-1 pages are not readily
>> available when the NIC is setup (usually one time per boot) is adding TLB cost
>> at run time, for billions of packets to come, maybe for months.
>>
>> Surely trying a bit harder to get order-1 pages is desirable.
>>
>>  __GFP_RETRY_MAYFAIL is supposed to help here.
> 
> Could we add __GFP_RETRY_MAYFAIL to kvmalloc, to ensure the allocation success ?

__GFP_RETRY_MAYFAIL does not _ensure_ the allocation success.

The idea here is that for large allocations (bigger than PAGE_SIZE),
kvmalloc_node() will not force __GFP_NORETRY, meaning that page allocator
will not bailout immediately in case of memory pressure.

This gives a chance for page reclaims to happen, and eventually the high order page
allocation will succeed under normal circumstances.

It is a trade-off, and only worth it for long living allocations.