Subject: Re: [PATCH 1/1] RDMA/mlx5: Release CPU for other processes in mlx5_free_cmd_msg()

From: Leon Romanovsky <[email-masked]>
Date: Fri, 31 May 2024 13:00:00 +0300

On Fri, May 31, 2024 at 10:21:39AM +0530, Anand Khoje wrote:
> 
> On 5/30/24 22:44, Leon Romanovsky wrote:
> > On Wed, May 22, 2024 at 09:02:56AM +0530, Anand Khoje wrote:
> > > In non FLR context, at times CX-5 requests release of ~8 million device pages.
> > > This needs humongous number of cmd mailboxes, which to be released once
> > > the pages are reclaimed. Release of humongous number of cmd mailboxes
> > > consuming cpu time running into many secs, with non preemptable kernels
> > > is leading to critical process starving on that cpu’s RQ. To alleviate
> > > this, this patch relinquishes cpu periodically but conditionally.
> > > 
> > > Orabug: 36275016
> > > 
> > > Signed-off-by: Anand Khoje <[email-masked]>
> > > ---
> > >   drivers/net/ethernet/mellanox/mlx5/core/cmd.c | 7 +++++++
> > >   1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/drivers/net/ethernet/mellanox/mlx5/core/cmd.c b/drivers/net/ethernet/mellanox/mlx5/core/cmd.c
> > > index 9c21bce..9fbf25d 100644
> > > --- a/drivers/net/ethernet/mellanox/mlx5/core/cmd.c
> > > +++ b/drivers/net/ethernet/mellanox/mlx5/core/cmd.c
> > > @@ -1336,16 +1336,23 @@ static struct mlx5_cmd_msg *mlx5_alloc_cmd_msg(struct mlx5_core_dev *dev,
> > >   	return ERR_PTR(err);
> > >   }
> > > +#define RESCHED_MSEC 2
> > >   static void mlx5_free_cmd_msg(struct mlx5_core_dev *dev,
> > >   			      struct mlx5_cmd_msg *msg)
> > >   {
> > >   	struct mlx5_cmd_mailbox *head = msg->next;
> > >   	struct mlx5_cmd_mailbox *next;
> > > +	unsigned long start_time = jiffies;
> > >   	while (head) {
> > >   		next = head->next;
> > >   		free_cmd_box(dev, head);
> > Did you consider to make this function asynchronous and parallel?
> > 
> > Thanks
> 
> Hi Leon,
> 
> Thanks for reviewing this patch.
> 
> Here, all page related methods give_pages/reclaim_pages/release_all_pages
> are executed in a worker thread through pages_work_handler().
> 
> Doesn't that mean it is already asynchronous?

You didn't provide any performance data, so I can't say if it is related to work_handlers.

For example, we can be in this loop when we call to mlx5_cmd_disable()
and it will cause to synchronous calls to dma_pool_free() which holds
the spinlock.

Also pages_work_handler() runs through single threaded workqueue, it is
not asynchronous.

> 
> When the worker thread, in this case it is processing reclaim_pages(), is
> taking a long time - it is starving other processes on the processor that it
> is running on. Oracle UEK being a non-preemptible kernel, these other
> processes that are getting starved do not get CPU until the worker
> relinquishes the CPU. This applies to even processes that are time critical
> and high priority. These processes when starved of CPU for a long time,
> trigger a kernel panic.

Please add kernel panic and perf data to your commit message.

> 
> Hence, this patch implements a time based relinquish of CPU using
> cond_resched().
> 
> Shay Dori, had a suggestion to tune the time (which we have made 2 msec), to
> reduce too frequent context switching and find a balance in processing of
> these mailbox objects. I am presently running some tests on the basis of
> this suggestion.

You will have better results if you parallel page release.

Thanks

> 
> Thanks,
> 
> Anand
> 
> > >   		head = next;
> > > +		if (time_after(jiffies, start_time + msecs_to_jiffies(RESCHED_MSEC))) {
> > > +			mlx5_core_warn_rl(dev, "Spent more than %d msecs, yielding CPU\n", RESCHED_MSEC);
> > > +			cond_resched();
> > > +			start_time = jiffies;
> > > +		}
> > >   	}
> > >   	kfree(msg);
> > >   }
> > > -- 
> > > 1.8.3.1
> > > 
> > >