Re: [PATCH net-next] net/mlx5: Reclaim max 50K pages at once
From: Anand Khoje
Date: Fri Jul 12 2024 - 00:49:15 EST
On 7/12/24 07:07, Saeed Mahameed wrote:
On 11 Jul 20:43, 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.
Where do you see this FW limit? currently we don't have it in the driver,
the driver requests from FW to reclaim exactly as many pages as the FW
already sent in the initial event. It is up to the FW to decide how many
pages out of those it actually release to the driver.
Hi Saeed,
We have a confirmation from Vendor i.e. nVidia that the current limit of
maximum pages firmware will release is 50K.
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
Sounds like you only release 50k pages out of the 1.3M! what happens
to the
rest? eventually we need to release them and waiting for driver unload
isn't an option.
My theory here of what happened before the patch:
1. FW: event to request to release 1.3M;
2. driver: prepare a FW command to release 1.3M, send it to FW with 1.3M;
3. FW: release 50K;
4. goto 1;
After the patch:
1. FW: event to request to release 1.3M;
2. driver: prepare a FW command to release 50k**, send it to FW with
50k*;
3. FW: release 50K; Driver didn't ask for more. no event required.
4. Done;
After your patch it seems like there 1.25M pages that are lingering in FW
ownership with no use.
We have tested this case, here are our observations:
1. FW: event to request to release 1.3M;
2. driver: prepare a FW command to release 50k**, send it to FW with 50k*;
3. FW: release 50K; Driver didn't ask for more. no event required.
4. goto 1 with 1.25M request to release in a new EQE.
It goes on till fw releases all pages from its ownership.
I hope that answers your doubt.
Thanks,
Anand
Signed-off-by: Anand Khoje <anand.a.khoje@xxxxxxxxxx>
Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxx>
Reviewed-by: Zhu Yanjun <yanjun.zhu@xxxxxxxxx>
---
drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c | 16
+++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
index d894a88..972e8e9 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
@@ -608,6 +608,11 @@ enum {
RELEASE_ALL_PAGES_MASK = 0x4000,
};
+/* This limit is based on the capability of the firmware as it
cannot release
+ * more than 50000 back to the host in one go.
+ */
+#define MAX_RECLAIM_NPAGES (-50000)
+
static int req_pages_handler(struct notifier_block *nb,
unsigned long type, void *data)
{
@@ -639,7 +644,16 @@ static int req_pages_handler(struct
notifier_block *nb,
req->dev = dev;
req->func_id = func_id;
- req->npages = npages;
+
+ /* npages > 0 means HCA asking host to allocate/give pages,
+ * npages < 0 means HCA asking host to reclaim back the pages
allocated.
+ * Here we are restricting the maximum number of pages that can be
+ * reclaimed to be MAX_RECLAIM_NPAGES. Note that
MAX_RECLAIM_NPAGES is
+ * a negative value.
+ * Since MAX_RECLAIM is negative, we are using max() to restrict
+ * req->npages (and not min ()).
+ */
+ req->npages = max_t(s32, npages, MAX_RECLAIM_NPAGES);
req->ec_function = ec_function;
req->release_all = release_all;
INIT_WORK(&req->work, pages_work_handler);
--
1.8.3.1