netdev - Re: [PATCH] mlx5: Remove call to ida_pre

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Date:   Tue, 20 Mar 2018 14:41:49 +0200
From:   Maor Gottlieb <maorg@...lanox.com>
To:     Saeed Mahameed <saeedm@...lanox.com>,
        "willy@...radead.org" <willy@...radead.org>
CC:     Matan Barak <matanb@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "leon@...nel.org" <leon@...nel.org>
Subject: Re: [PATCH] mlx5: Remove call to ida_pre_get



On 3/20/2018 5:29 AM, Saeed Mahameed wrote:
> On Thu, 2018-03-15 at 18:30 -0700, Matthew Wilcox wrote:
>> On Thu, Mar 15, 2018 at 11:58:07PM +0000, Saeed Mahameed wrote:
>>> On Wed, 2018-03-14 at 19:57 -0700, Matthew Wilcox wrote:
>>>> From: Matthew Wilcox <mawilcox@...rosoft.com>
>>>>
>>>> The mlx5 driver calls ida_pre_get() in a loop for no readily
>>>> apparent
>>>> reason.  The driver uses ida_simple_get() which will call
>>>> ida_pre_get()
>>>> by itself and there's no need to use ida_pre_get() unless using
>>>> ida_get_new().
>>>>
>>> Hi Matthew,
>>>
>>> Is this is causing any issues ? or just a simple cleanup ?
>> I'm removing the API.  At the end of this cleanup, there will be no
>> more
>> preallocation; instead we will rely on the slab allocator not
>> sucking.
>>
> Ok, Seems reasonable, I am ok with this.
>
>>> Adding Maor, the author of this change,
>>>
>>> I believe the idea is to speed up insert_fte (which calls
>>> ida_simple_get) since insert_fte runs under the FTE write
>>> semaphore,
>>> in this case if ida_pre_get was successful before taking the
>>> semaphore
>>> for all the FTE nodes in the loop, this will be a huge win for
>>> ida_simple_get which will immediately return success without even
>>> trying to allocate.
>> I think that's misguided.  The IDA allocator is only going to
>> allocate
>> memory once in every 1024 allocations.  Also, it does try to
>> allocate,
>> even if there are preallocated nodes.  So you're just wasting time,
>> unfortunately.
>>
> Well just by looking at the code you can tell for sure that
> two consecutive calls to ida_pre_get will result in one allocation
> only.
> due to "if (!this_cpu_read(ida_bitmap))"
>
> but i didn't dig into details and didn't go through the whole
> ida_get_new_above, so i will count on your judgment here.
>
> Still i would like to wait for Maor's input here, the author..
> I Will ping him today.
>
> Thanks,
> Saeed.

Saeed, Matan and I okay with this fix as well, it looks like it 
shouldn't impact on the insertion rate.