netdev - Re: [PATCH net-next] net/mlx5: Make ASO poll CQ usable in atomic context

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Date:   Sat, 24 Sep 2022 13:11:31 -0700
From:   Saeed Mahameed <saeedm@...dia.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next] net/mlx5: Make ASO poll CQ usable in atomic
 context

On 24 Sep 21:51, Leon Romanovsky wrote:
>On Sat, Sep 24, 2022 at 10:24:25AM -0700, Saeed Mahameed wrote:
>> On 24 Sep 14:17, Leon Romanovsky wrote:
>> > From: Leon Romanovsky <leonro@...dia.com>
>> >
>> > Poll CQ functions shouldn't sleep as they are called in atomic context.
>> > The following splat appears once the mlx5_aso_poll_cq() is used in such
>> > flow.
>> >
>> > BUG: scheduling while atomic: swapper/17/0/0x00000100
>>
>> ...
>>
>> > 	/* With newer FW, the wait for the first ASO WQE is more than 2us, put the wait 10ms. */
>> > -	err = mlx5_aso_poll_cq(aso, true, 10);
>> > +	expires = jiffies + msecs_to_jiffies(10);
>> > +	do {
>> > +		err = mlx5_aso_poll_cq(aso, true);
>> > +	} while (err && time_is_after_jiffies(expires));
>> > 	mutex_unlock(&flow_meters->aso_lock);
>>         ^^^^
>> busy poll won't work, this mutex is held and can sleep anyway.
>> Let's discuss internally and solve this by design.
>
>This is TC code, it doesn't need atomic context and had mutex + sleep
>from the beginning.
>

then let's bring back the sleep for TC use-case, we don't need to burn the
CPU!
But still the change has bigger effect on other aso use cases, see below.

>My change cleans mlx5_aso_poll_cq() from busy loop for the IPsec path,
>so why do you plan to change in the design?
>

a typical use case for aso is

post_aso_wqe();
poll_aso_cqe();

The HW needs time to consume and excute the wqe then generate the CQE.
This is why the driver needs to wait a bit when polling for the cqe,
your change will break others. Let's discuss internally.

>Thanks