Re: [PATCH for-next 03/11] IB/hns: Optimize the logic of allocating memory using APIs

On Mon, Nov 21, 2016 at 04:12:38PM +0000, Salil Mehta wrote:

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-----Original Message-----
From: Leon Romanovsky [mailto:leon@kernel.org]
Sent: Wednesday, November 16, 2016 8:36 AM
To: Salil Mehta
Cc: dledford@redhat.com; Huwei (Xavier); oulijun;
mehta.salil.lnk@gmail.com; linux-rdma@vger.kernel.org;
netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Linuxarm;
Zhangping (ZP)
Subject: Re: [PATCH for-next 03/11] IB/hns: Optimize the logic of
allocating memory using APIs

On Tue, Nov 15, 2016 at 03:52:46PM +0000, Salil Mehta wrote:

quoted

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-----Original Message-----
From: Leon Romanovsky [mailto:leon@kernel.org]
Sent: Wednesday, November 09, 2016 7:22 AM
To: Salil Mehta
Cc: dledford@redhat.com; Huwei (Xavier); oulijun;
mehta.salil.lnk@gmail.com; linux-rdma@vger.kernel.org;
netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Linuxarm;
Zhangping (ZP)
Subject: Re: [PATCH for-next 03/11] IB/hns: Optimize the logic of
allocating memory using APIs

On Fri, Nov 04, 2016 at 04:36:25PM +0000, Salil Mehta wrote:

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From: "Wei Hu (Xavier)" <redacted>

This patch modified the logic of allocating memory using APIs in
hns RoCE driver. We used kcalloc instead of kmalloc_array and
bitmap_zero. And When kcalloc failed, call vzalloc to alloc
memory.

Signed-off-by: Wei Hu (Xavier) <redacted>
Signed-off-by: Ping Zhang <redacted>
Signed-off-by: Salil Mehta  <redacted>
---
 drivers/infiniband/hw/hns/hns_roce_mr.c |   15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/infiniband/hw/hns/hns_roce_mr.c

b/drivers/infiniband/hw/hns/hns_roce_mr.c

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index fb87883..d3dfb5f 100644

--- a/drivers/infiniband/hw/hns/hns_roce_mr.c
+++ b/drivers/infiniband/hw/hns/hns_roce_mr.c

@@ -137,11 +137,12 @@ static int hns_roce_buddy_init(struct

hns_roce_buddy *buddy, int max_order)

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 	for (i = 0; i <= buddy->max_order; ++i) {
 		s = BITS_TO_LONGS(1 << (buddy->max_order - i));
-		buddy->bits[i] = kmalloc_array(s, sizeof(long),

GFP_KERNEL);

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-		if (!buddy->bits[i])
-			goto err_out_free;
-
-		bitmap_zero(buddy->bits[i], 1 << (buddy->max_order -

i));

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+		buddy->bits[i] = kcalloc(s, sizeof(long),

GFP_KERNEL);

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+		if (!buddy->bits[i]) {
+			buddy->bits[i] = vzalloc(s * sizeof(long));

I wonder, why don't you use directly vzalloc instead of kcalloc
fallback?

As we know we will have physical contiguous pages if the kcalloc
call succeeds. This will give us a chance to have better performance
over the allocations which are just virtually contiguous through the
function vzalloc(). Therefore, later has only been used as a fallback
when our memory request cannot be entertained through kcalloc.

Are you suggesting that there will not be much performance penalty
if we use just vzalloc ?

Not exactly,
I asked it, because we have similar code in our drivers and this
construction looks strange to me.

1. If performance is critical, we will use kmalloc.
2. If performance is not critical, we will use vmalloc.

But in this case, such construction shows me that we can live with
vmalloc performance and kmalloc allocation are not really needed.

In your specific case, I'm not sure that kcalloc will ever fail.

Performance is definitely critical here. Though, I agree this is bit
unusual way of memory allocation. In actual, we were encountering
memory alloc failures using kmalloc (if you see allocation amount
is on the higher side and is exponential) so we ended up using
vmalloc as fall back - It is very naïve allocation scheme.

I understand it, we did the same, see our mlx5_vzalloc call.
BTW, we used __GFP_NOWARN flag, which you should consider to use
in your case too.

Maybe we need to rethink this allocation scheme part? Also, I can pull
back this particular patch for now or just live with vzalloc() till
we figure out proper solution to this?

It is up to you, I don't think that you should drop it, AFAIK, there is
no other proper solution.

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Thanks

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+			if (!buddy->bits[i])
+				goto err_out_free;
+		}
 	}