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Message-ID:
<CY8PR12MB7195E405C3EC9F43619231CCDC4B2@CY8PR12MB7195.namprd12.prod.outlook.com>
Date: Tue, 29 Oct 2024 04:08:21 +0000
From: Parav Pandit <parav@...dia.com>
To: Caleb Sander Mateos <csander@...estorage.com>, Saeed Mahameed
<saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>, Tariq Toukan
<tariqt@...dia.com>, Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] mlx5: only schedule EQ comp tasklet if necessary
Hi
> From: Caleb Sander Mateos <csander@...estorage.com>
> Sent: Sunday, October 27, 2024 9:37 AM
>
> Currently, the mlx5_eq_comp_int() interrupt handler schedules a tasklet to call
> mlx5_cq_tasklet_cb() if it processes any completions. For CQs whose
> completions don't need to be processed in tasklet context, this overhead is
> unnecessary. Atomic operations are needed to schedule, lock, and clear the
> tasklet. And when mlx5_cq_tasklet_cb() runs, it acquires a spin lock to access
> the list of CQs enqueued for processing.
>
> Schedule the tasklet in mlx5_add_cq_to_tasklet() instead to avoid this
> overhead. mlx5_add_cq_to_tasklet() is responsible for enqueuing the CQs to
> be processed in tasklet context, so it can schedule the tasklet. CQs that need
> tasklet processing have their interrupt comp handler set to
> mlx5_add_cq_to_tasklet(), so they will schedule the tasklet. CQs that don't
> need tasklet processing won't schedule the tasklet. To avoid scheduling the
> tasklet multiple times during the same interrupt, only schedule the tasklet in
> mlx5_add_cq_to_tasklet() if the tasklet work queue was empty before the
> new CQ was pushed to it.
>
> Note that the mlx4 driver works the same way: it schedules the tasklet in
> mlx4_add_cq_to_tasklet() and only if the work queue was empty before.
>
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> ---
> drivers/net/ethernet/mellanox/mlx5/core/cq.c | 5 +++++
> drivers/net/ethernet/mellanox/mlx5/core/eq.c | 5 +----
> 2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/cq.c
> b/drivers/net/ethernet/mellanox/mlx5/core/cq.c
> index 4caa1b6f40ba..25f3b26db729 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/cq.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/cq.c
> @@ -69,22 +69,27 @@ void mlx5_cq_tasklet_cb(struct tasklet_struct *t)
> static void mlx5_add_cq_to_tasklet(struct mlx5_core_cq *cq,
> struct mlx5_eqe *eqe)
> {
> unsigned long flags;
> struct mlx5_eq_tasklet *tasklet_ctx = cq->tasklet_ctx.priv;
> + bool schedule_tasklet = false;
>
> spin_lock_irqsave(&tasklet_ctx->lock, flags);
> /* When migrating CQs between EQs will be implemented, please note
> * that you need to sync this point. It is possible that
> * while migrating a CQ, completions on the old EQs could
> * still arrive.
> */
> if (list_empty_careful(&cq->tasklet_ctx.list)) {
> mlx5_cq_hold(cq);
> + schedule_tasklet = list_empty(&tasklet_ctx->list);
> list_add_tail(&cq->tasklet_ctx.list, &tasklet_ctx->list);
> }
> spin_unlock_irqrestore(&tasklet_ctx->lock, flags);
> +
> + if (schedule_tasklet)
> + tasklet_schedule(&tasklet_ctx->task);
> }
>
> /* Callers must verify outbox status in case of err */ int mlx5_create_cq(struct
> mlx5_core_dev *dev, struct mlx5_core_cq *cq,
> u32 *in, int inlen, u32 *out, int outlen) diff --git
> a/drivers/net/ethernet/mellanox/mlx5/core/eq.c
> b/drivers/net/ethernet/mellanox/mlx5/core/eq.c
> index 68cb86b37e56..66fc17d9c949 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/eq.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/eq.c
> @@ -112,17 +112,17 @@ static int mlx5_eq_comp_int(struct notifier_block
> *nb,
> struct mlx5_eq_comp *eq_comp =
> container_of(nb, struct mlx5_eq_comp, irq_nb);
> struct mlx5_eq *eq = &eq_comp->core;
> struct mlx5_eqe *eqe;
> int num_eqes = 0;
> - u32 cqn = -1;
>
> eqe = next_eqe_sw(eq);
> if (!eqe)
> goto out;
>
> do {
> + u32 cqn;
> struct mlx5_core_cq *cq;
>
A small nit, cqn should be declared after cq to follow the netdev coding guidelines of [1].
> /* Make sure we read EQ entry contents after we've
> * checked the ownership bit.
> */
> @@ -145,13 +145,10 @@ static int mlx5_eq_comp_int(struct notifier_block
> *nb,
> } while ((++num_eqes < MLX5_EQ_POLLING_BUDGET) && (eqe =
> next_eqe_sw(eq)));
>
> out:
> eq_update_ci(eq, 1);
>
> - if (cqn != -1)
> - tasklet_schedule(&eq_comp->tasklet_ctx.task);
> -
Current code processes many EQEs and performs the check for tasklet_schedule only once in the cqn check.
While this change, on every EQE, the additional check will be done.
This will marginally make the interrupt handler slow.
Returning a bool from comp() wont be good either, and we cannot inline things here due to function pointer.
The cost of scheduling null tasklet is higher than this if (schedule_tasklet) check.
In other series internally, I am working to reduce the cost of comp() itself unrelated to this change.
so it ok to have the additional check introduced here.
Apart from that,
Reviewed-by: Parav Pandit <parav@...dia.com>
[1] https://elixir.bootlin.com/linux/v6.11.5/source/Documentation/process/maintainer-netdev.rst#L358
> return 0;
> }
>
> /* Some architectures don't latch interrupts when they are disabled, so using
> * mlx5_eq_poll_irq_disabled could end up losing interrupts while trying to
> --
> 2.45.2
>
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