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

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Message-ID: <1521516573.32324.63.camel@mellanox.com>
Date:   Tue, 20 Mar 2018 03:29:36 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "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>,
        Maor Gottlieb <maorg@...lanox.com>,
        "leon@...nel.org" <leon@...nel.org>
Subject: Re: [PATCH] mlx5: Remove call to ida_pre_get

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.