linux-kernel - Re: [PATCH net-next v3] mlx5/core: Schedule EQ comp tasklet only if necessary

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Message-ID: <cf2d112f-7888-4e36-8212-d8c632fd323d@redhat.com>
Date: Tue, 5 Nov 2024 13:21:46 +0100
From: Paolo Abeni <pabeni@...hat.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>
Cc: Parav Pandit <parav@...dia.com>, netdev@...r.kernel.org,
 linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3] mlx5/core: Schedule EQ comp tasklet only if
 necessary

On 10/31/24 17:34, Caleb Sander Mateos wrote:
> 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
> adds unnecessary overhead. In a heavy TCP workload, we see 4% of CPU
> time spent on the tasklet_trylock() in tasklet_action_common(), with a
> smaller amount spent on the atomic operations in tasklet_schedule(),
> tasklet_clear_sched(), and locking the spinlock in mlx5_cq_tasklet_cb().
> TCP completions are handled by mlx5e_completion_event(), which schedules
> NAPI to poll the queue, so they don't need tasklet 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.
> 
> The additional branch in mlx5_add_cq_to_tasklet(), called for each EQE,
> may add a small cost for the userspace Infiniband CQs whose completions
> are processed in tasklet context. But this seems worth it to avoid the
> tasklet overhead for CQs that don't need 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>
> Reviewed-by: Parav Pandit <parav@...dia.com>

@Saeed, @Leon, @Tariq: I assume you will apply this one and include in
the next mlx5 PR. please correct me if I'm wrong.

Thanks,

Paolo