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Date:   Wed, 30 May 2018 14:22:56 +0200
From:   Hans Westgaard Ry <hans.westgaard.ry@...cle.com>
To:     jackm <jackm@....mellanox.co.il>, Jason Gunthorpe <jgg@...pe.ca>
Cc:     Håkon Bugge <haakon.bugge@...cle.com>,
        Doug Ledford <dledford@...hat.com>,
        Daniel Jurgens <danielj@...lanox.com>,
        Parav Pandit <parav@...lanox.com>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        OFED mailing list <linux-rdma@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Leon Romanovsky <leon@...nel.org>
Subject: Re: [PATCH] IB/mad: Use ID allocator routines to allocate agent
 number



On 05/30/2018 10:02 AM, jackm wrote:
> On Tue, 29 May 2018 10:40:32 -0600
> Jason Gunthorpe <jgg@...pe.ca> wrote:
>
>> On Tue, May 29, 2018 at 06:16:14PM +0200, Håkon Bugge wrote:
>>>   
>>>> On 29 May 2018, at 17:49, Jason Gunthorpe <jgg@...pe.ca> wrote:
>>>>
>>>> On Tue, May 29, 2018 at 09:38:08AM +0200, Hans Westgaard Ry
>>>> wrote:
>>>>> The agent TID is a 64 bit value split in two dwords.  The least
>>>>> significant dword is the TID running counter. The most
>>>>> significant dword is the agent number. In the CX-3 shared port
>>>>> model, the mlx4 driver uses the most significant byte of the
>>>>> agent number to store the slave number, making agent numbers
>>>>> greater and equal to 2^24 (3 bytes) unusable.
>>>> There is no reason for this to be an ida, just do something like
>>>>
>>>> mad_agent_priv->agent.hi_tid =
>>>> atomic_inc_return(&ib_mad_client_id) &
>>>> mad_agent_priv->ib_dev->tid_mask;
>>>>
>>>> And have the driver set tid_mask to 3 bytes of 0xFF
>>> The issue is that some of the mad agents are long-lived, so you will
>>> wrap and use the same TID twice.
>> We already have that problem, and using ida is problematic because we
>> need to maximize the time between TID re-use, which ida isn't doing.
>>
>> Preventing re-use seems like a seperate issue from limiting the range
>> to be compatible with mlx4.
>>
> Preventing immediate re-use can be accomplished by judicious use of the
> start argument (second argument) in the call to ida_simple_get (to
> introduce hysteresis into the id allocations).
>
> For example, can do something like:
>
> static atomic_t ib_mad_client_id_min = ATOMIC_INIT(1);
> ...
>          ib_mad_client_id = ida_simple_get(&ib_mad_client_ids,
>                                            atomic_read(&ib_mad_client_id_min),
>                                            ib_mad_sysctl_client_id_max,
>                                            GFP_KERNEL);
> ....
>          if (!(ib_mad_client_id % 1000) ||
>              ib_mad_sysctl_client_id_max - ib_mad_client_id <= 1000)
> 		atomic_set(&ib_mad_client_id_min, 1);
> 	else
>                  atomic_set(&ib_mad_client_id_min, ib_mad_client_id + 1);
>
> The above avoids immediate re-use of ids, and only searches for past
> freed ids if the last allocated-id is zero mod 1000.
>
> This is just suggestion -- will probably need some variation of the
> above to handle what happens over time (i.e., to not depend on the
> modulo operation to reset the search start to 1), to properly handle
> how we deal with the start value when we are close to the allowed
> client_id_max, and also to implement some sort of locking.
>
> -Jack
>
We came up with this code snippet which we think handles both preventing 
immediate re-use and too big/wrapping...

         max = mad_agent_priv->ib_dev->tid_max;
         start = atomic_inc_return(&start);
retry:
         tid = ida_simple_get(&ib_mad_client_ids, start, max, GFP_KERNEL);
         if (unlikely(tid == -ENOSPC))    {
                 spin_lock_irq_save();
                 tid = ida_simple_get(&ib_mad_client_ids, start, max, 
GFP_ATOMIC);
                 if (unlikely(tid == -ENOSPC))    {
                         atomic_set(&start, 1);
                         spin_unlock_irq_restore();
                         goto retry;
                 }
                 spin_unlock_irq_restore();
         }


Hans


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