Re: [PATCH v2] RDMA/mlx5 : Reclaim max 50K pages at once

[Anand Khoje <anand.a.khoje@oracle.com>]

On 6/14/24 00:33, Leon Romanovsky wrote:
> On Thu, Jun 13, 2024 at 05:42:52PM +0530, Anand Khoje wrote:
>> In non FLR context, at times CX-5 requests release of ~8 million FW pages.
>> This needs humongous number of cmd mailboxes, which to be released once
>> the pages are reclaimed. Release of humongous number of cmd mailboxes is
>> consuming cpu time running into many seconds. Which with non preemptible
>> kernels is leading to critical process starving on that cpu’s RQ.
>> To alleviate this, this change restricts the total number of pages
>> a worker will try to reclaim maximum 50K pages in one go.
>> The limit 50K is aligned with the current firmware capacity/limit of
>> releasing 50K pages at once per MLX5_CMD_OP_MANAGE_PAGES + MLX5_PAGES_TAKE
>> device command.
>>
>> Our tests have shown significant benefit of this change in terms of
>> time consumed by dma_pool_free().
>> During a test where an event was raised by HCA
>> to release 1.3 Million pages, following observations were made:
>>
>> - Without this change:
>> Number of mailbox messages allocated was around 20K, to accommodate
>> the DMA addresses of 1.3 million pages.
>> The average time spent by dma_pool_free() to free the DMA pool is between
>> 16 usec to 32 usec.
>>             value  ------------- Distribution ------------- count
>>               256 |                                         0
>>               512 |@                                        287
>>              1024 |@@@                                      1332
>>              2048 |@                                        656
>>              4096 |@@@@@                                    2599
>>              8192 |@@@@@@@@@@                               4755
>>             16384 |@@@@@@@@@@@@@@@                          7545
>>             32768 |@@@@@                                    2501
>>             65536 |                                         0
>>
>> - With this change:
>> Number of mailbox messages allocated was around 800; this was to
>> accommodate DMA addresses of only 50K pages.
>> The average time spent by dma_pool_free() to free the DMA pool in this case
>> lies between 1 usec to 2 usec.
>>             value  ------------- Distribution ------------- count
>>               256 |                                         0
>>               512 |@@@@@@@@@@@@@@@@@@                       346
>>              1024 |@@@@@@@@@@@@@@@@@@@@@@                   435
>>              2048 |                                         0
>>              4096 |                                         0
>>              8192 |                                         1
>>             16384 |                                         0
>>
>> Signed-off-by: Anand Khoje <anand.a.khoje@oracle.com>
>> ---
>> Changes in v2:
>>   - In v1, CPUs were yielded if more than 2 msec are spent in
>>     mlx5_free_cmd_msg(). The approach to limit the time spent is changed
>>     in this version.
>> ---
>>   drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
>> index 1b38397..b1cf97d 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
>> @@ -482,12 +482,16 @@ static int reclaim_pages(struct mlx5_core_dev *dev, u32 func_id, int npages,
>>   	return err;
>>   }
>>   
>> +#define MAX_RECLAIM_NPAGES -50000
>>   static void pages_work_handler(struct work_struct *work)
>>   {
>>   	struct mlx5_pages_req *req = container_of(work, struct mlx5_pages_req, work);
>>   	struct mlx5_core_dev *dev = req->dev;
>>   	int err = 0;
>>   
>> +	if (req->npages < MAX_RECLAIM_NPAGES)
>> +		req->npages = MAX_RECLAIM_NPAGES;
> I like this change more than previous variant with yield.
> Regarding the patch:
> 1. Please limit the number of pages in req_pages_handler() and not int pages_work_handler().
> 2. Patch title should be "net/mlx5: Reclaim max 50K pages at once" and not "RDMA...".
> 3. You should run get_maintainer.pl script to find the right maintainers and add them to the TO or CC list.
>
> And I still think that you will get better performance by parallelizing the reclaim process.
>
> Thanks

Hi Leon,

Thanks for the comments. I will make the changes and resend a v3.

-Anand

>> +
>>   	if (req->release_all)
>>   		release_all_pages(dev, req->func_id, req->ec_function);
>>   	else if (req->npages < 0)
>> -- 
>> 1.8.3.1
>>
>>