netdev - Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

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Date:   Wed, 15 Feb 2017 15:59:18 +0200
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Matan Barak <matanb@...lanox.com>, jackm@...lanox.com
Subject: Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

On Wed, Feb 15, 2017 at 3:29 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-02-15 at 13:10 +0200, Saeed Mahameed wrote:
>> On Fri, Feb 10, 2017 at 2:27 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > From: Eric Dumazet <edumazet@...gle.com>
>> >
>> > All rx and rx netdev interrupts are handled by respectively
>> > by mlx4_en_rx_irq() and mlx4_en_tx_irq() which simply schedule a NAPI.
>> >
>> > But mlx4_eq_int() also fires a tasklet to service all items that were
>> > queued via mlx4_add_cq_to_tasklet(), but this handler was not called
>> > unless user cqe was handled.
>> >
>> > This is very confusing, as "mpstat -I SCPU ..." show huge number of
>> > tasklet invocations.
>> >
>> > This patch saves this overhead, by carefully firing the tasklet directly
>> > from mlx4_add_cq_to_tasklet(), removing four atomic operations per IRQ.
>> >
>> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>> > Cc: Tariq Toukan <tariqt@...lanox.com>
>> > Cc: Saeed Mahameed <saeedm@...lanox.com>
>> > ---
>> >  drivers/net/ethernet/mellanox/mlx4/cq.c |    6 +++++-
>> >  drivers/net/ethernet/mellanox/mlx4/eq.c |    9 +--------
>> >  2 files changed, 6 insertions(+), 9 deletions(-)
>> >
>> > diff --git a/drivers/net/ethernet/mellanox/mlx4/cq.c b/drivers/net/ethernet/mellanox/mlx4/cq.c
>> > index 6b8635378f1fcb2aae4e8ac390bcd09d552c2256..fa6d2354a0e910ee160863e3cbe21a512d77bf03 100644
>> > --- a/drivers/net/ethernet/mellanox/mlx4/cq.c
>> > +++ b/drivers/net/ethernet/mellanox/mlx4/cq.c
>> > @@ -81,8 +81,9 @@ void mlx4_cq_tasklet_cb(unsigned long data)
>> >
>> >  static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
>> >  {
>> > -       unsigned long flags;
>> >         struct mlx4_eq_tasklet *tasklet_ctx = cq->tasklet_ctx.priv;
>> > +       unsigned long flags;
>> > +       bool kick;
>> >
>> >         spin_lock_irqsave(&tasklet_ctx->lock, flags);
>> >         /* When migrating CQs between EQs will be implemented, please note
>> > @@ -92,7 +93,10 @@ static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
>> >          */
>> >         if (list_empty_careful(&cq->tasklet_ctx.list)) {
>> >                 atomic_inc(&cq->refcount);
>> > +               kick = list_empty(&tasklet_ctx->list);
>>
>> So first one in would fire the tasklet, but wouldn't this cause CQE
>> processing loss
>> in the same mlx4_eq_int loop if the tasklet was fast enough to
>> schedule and while other CQEs are going to add themselves to the
>> tasklet_ctx->list ?
>
>
> mlx4_eq_int() is a hard irq handler.
>
> How a tasklet could run in the middle of it ?
>

can the tasklet run on a different core ?

> A tasklet is a softirq handler.
>
> softirq must wait that the current hard irq handler is done.
>>
>> Anyway i tried to find race scenarios that could cause such thing but
>> synchronization looks good.
>>
>> >                 list_add_tail(&cq->tasklet_ctx.list, &tasklet_ctx->list);
>> > +               if (kick)
>> > +                       tasklet_schedule(&tasklet_ctx->task);
>> >         }
>> >         spin_unlock_irqrestore(&tasklet_ctx->lock, flags);
>> >  }
>> > diff --git a/drivers/net/ethernet/mellanox/mlx4/eq.c b/drivers/net/ethernet/mellanox/mlx4/eq.c
>> > index 0509996957d9664b612358dd805359f4bc67b8dc..39232b6a974f4b4b961d3b0b8634f04e6b9d0caa 100644
>> > --- a/drivers/net/ethernet/mellanox/mlx4/eq.c
>> > +++ b/drivers/net/ethernet/mellanox/mlx4/eq.c
>> > @@ -494,7 +494,7 @@ static int mlx4_eq_int(struct mlx4_dev *dev, struct mlx4_eq *eq)
>> >  {
>> >         struct mlx4_priv *priv = mlx4_priv(dev);
>> >         struct mlx4_eqe *eqe;
>> > -       int cqn = -1;
>> > +       int cqn;
>> >         int eqes_found = 0;
>> >         int set_ci = 0;
>> >         int port;
>> > @@ -840,13 +840,6 @@ static int mlx4_eq_int(struct mlx4_dev *dev, struct mlx4_eq *eq)
>> >
>> >         eq_set_ci(eq, 1);
>> >
>> > -       /* cqn is 24bit wide but is initialized such that its higher bits
>> > -        * are ones too. Thus, if we got any event, cqn's high bits should be off
>> > -        * and we need to schedule the tasklet.
>> > -        */
>> > -       if (!(cqn & ~0xffffff))
>>
>> what if we simply change this condition to:
>> if (!list_empty_careful(eq->tasklet_ctx.list))
>>
>> Wouldn't this be sort of equivalent to what you did ? and this way we
>> would simply fire the tasklet only when needed and not on every
>> handled CQE.
>
> Still this test would be done one million time per second on my hosts.
>
> What is the point exactly ?
>

the point is that if the EQ is full of CQEs from different CQs you would
do the "  kick = list_empty(&tasklet_ctx->list);" test per empty CQ
list rather than once at the end.

in mlx4_en case, you have only two CQs on each EQ but in RoCE/IB you
can have as many CQs as you want.

> Thanks.
>
>