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Date:   Sun, 20 Feb 2022 11:32:30 +0000
From:   Biju Das <biju.das.jz@...renesas.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Sergey Shtylyov <s.shtylyov@....ru>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>
Subject: RE: [PATCH] ravb: Use GFP_KERNEL instead of GFP_ATOMIC when possible

Hi Christophe,

Thanks for your clarification.

Patch Looks good to me.

Reviewed-by: Biju Das <biju.das.jz@...renesas.com>

Cheers,
Biju

> -----Original Message-----
> From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> Sent: 20 February 2022 08:49
> To: Biju Das <biju.das.jz@...renesas.com>; Sergey Shtylyov
> <s.shtylyov@....ru>; David S. Miller <davem@...emloft.net>; Jakub Kicinski
> <kuba@...nel.org>
> Cc: linux-kernel@...r.kernel.org; kernel-janitors@...r.kernel.org;
> netdev@...r.kernel.org; linux-renesas-soc@...r.kernel.org
> Subject: Re: [PATCH] ravb: Use GFP_KERNEL instead of GFP_ATOMIC when
> possible
> 
> Le 20/02/2022 à 08:53, Biju Das a écrit :
> > Hi Christophe,
> >
> > Thanks for the patch.
> >
> > Just a  question, As per [1], former can be allocated from interrupt
> context.
> > But nothing mentioned for the allocation using the patch you
> > mentioned[2]. I agree GFP_KERNEL gives more opportunities of successful
> allocation.
> 
> Hi,
> 
> netdev_alloc_skb() uses an implicit GFP_ATOMIC, that is why it can be
> safely called from an interrupt context.
> __netdev_alloc_skb() is the same as netdev_alloc_skb(), except that you
> can choose the GFP flag you want to use. ([1])
> 
> Here, the netdev_alloc_skb() is called just after some
> "kcalloc(GFP_KERNEL);"
> 
> So this function can already NOT be called from interrupt context.
> 
> So if GFP_KERNEL is fine here for kcalloc(), it is fine also for
> netdev_alloc_skb(), hence __netdev_alloc_skb(GFP_KERNEL).
> 
> >
> > Q1) Here it allocates 8K instead of 1K on each loop, Is there any
> limitation for netdev_alloc_skb for allocating 8K size?
> 
> Not sure to understand.
> My patch does NOT change anything on the amount of memory allocated. it
> only changes a GFP_ATOMIC into a GFP_KERNEL.
> 
> I'm not aware of specific limitation for netdev_alloc_skb().
> My understanding is that in the worst case, it will behave just like
> malloc() ([3])
> 
> So, if it was an issue before, it is still an issue after my patch.
> 
> > Q2) In terms of allocation performance which is better netdev_alloc_skb
> or __netdev_alloc_skb?
> 
> AFAIK, there should be no difference, but __netdev_alloc_skb(GFP_KERNEL)
> can succeed where netdev_alloc_skb() can fail. In such a case, it would be
> slower but most importantly, it would succeed.
> 
> 
> CJ
> 
> [1]:
> https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.b
> ootlin.com%2Flinux%2Fv5.17-
> rc4%2Fsource%2Finclude%2Flinux%2Fskbuff.h%23L2945&amp;data=04%7C01%7Cbiju.
> das.jz%40bp.renesas.com%7C5fab03422ea84da79f5308d9f44dd4fd%7C53d82571da194
> 7e49cb4625a166a4a2a%7C0%7C0%7C637809437402718622%7CUnknown%7CTWFpbGZsb3d8e
> yJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&a
> mp;sdata=0GC7vUwDkz5n7wESWC2gK3x6e8RalA%2FCg%2FmokTv%2BIHE%3D&amp;reserved
> =0
> 
> [2]:
> https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.b
> ootlin.com%2Flinux%2Fv5.17-
> rc4%2Fsource%2Fdrivers%2Fnet%2Fethernet%2Frenesas%2Fravb_main.c%23L470&amp
> ;data=04%7C01%7Cbiju.das.jz%40bp.renesas.com%7C5fab03422ea84da79f5308d9f44
> dd4fd%7C53d82571da1947e49cb4625a166a4a2a%7C0%7C0%7C637809437402718622%7CUn
> known%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLC
> JXVCI6Mn0%3D%7C3000&amp;sdata=KsLMiap6E%2BBEl4DOqBvPE%2BVsfCtTpZqAa4PvyKFq
> B9E%3D&amp;reserved=0
> 
> [3]:
> https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.b
> ootlin.com%2Flinux%2Fv5.17-
> rc3%2Fsource%2Fnet%2Fcore%2Fskbuff.c%23L488&amp;data=04%7C01%7Cbiju.das.jz
> %40bp.renesas.com%7C5fab03422ea84da79f5308d9f44dd4fd%7C53d82571da1947e49cb
> 4625a166a4a2a%7C0%7C0%7C637809437402718622%7CUnknown%7CTWFpbGZsb3d8eyJWIjo
> iMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sda
> ta=kwR2VU5sT%2FiD9y5VUMTztet1btFjlHqU1j5pCXF%2F1Vk%3D&amp;reserved=0
> 
> 
> >
> > [1]
> > https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.
> > kernel.org%2Fdoc%2Fhtmldocs%2Fnetworking%2FAPI-netdev-alloc-skb.html&a
> > mp;data=04%7C01%7Cbiju.das.jz%40bp.renesas.com%7C5fab03422ea84da79f530
> > 8d9f44dd4fd%7C53d82571da1947e49cb4625a166a4a2a%7C0%7C0%7C6378094374027
> > 18622%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJB
> > TiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=AhBE6136UO98boBjSE3bpBSDv8
> > 2EsuvgzXbOHy%2FIM1U%3D&amp;reserved=0
> > [2]
> > https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.
> > kernel.org%2Fdoc%2Fhtmldocs%2Fnetworking%2FAPI---netdev-alloc-skb.html
> > &amp;data=04%7C01%7Cbiju.das.jz%40bp.renesas.com%7C5fab03422ea84da79f5
> > 308d9f44dd4fd%7C53d82571da1947e49cb4625a166a4a2a%7C0%7C0%7C63780943740
> > 2718622%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLC
> > JBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=IqNfl5kZoQzO%2FB%2Frfq7q
> > 4QbgxKzoCy6iWB1ZXER7zO4%3D&amp;reserved=0
> >
> > Regards,
> > Biju
> >
> >> Subject: [PATCH] ravb: Use GFP_KERNEL instead of GFP_ATOMIC when
> >> possible
> >>
> >> 'max_rx_len' can be up to GBETH_RX_BUFF_MAX (i.e. 8192) (see
> >> 'gbeth_hw_info').
> >> The default value of 'num_rx_ring' can be BE_RX_RING_SIZE (i.e. 1024).
> >>
> >> So this loop can allocate 8 Mo of memory.
> >>
> >> Previous memory allocations in this function already use GFP_KERNEL,
> >> so use __netdev_alloc_skb() and an explicit GFP_KERNEL instead of a
> >> implicit GFP_ATOMIC.
> >>
> >> This gives more opportunities of successful allocation.
> >>
> >> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> >> ---
> >>   drivers/net/ethernet/renesas/ravb_main.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/ethernet/renesas/ravb_main.c
> >> b/drivers/net/ethernet/renesas/ravb_main.c
> >> index 24e2635c4c80..525d66f71f02 100644
> >> --- a/drivers/net/ethernet/renesas/ravb_main.c
> >> +++ b/drivers/net/ethernet/renesas/ravb_main.c
> >> @@ -475,7 +475,7 @@ static int ravb_ring_init(struct net_device
> >> *ndev, int
> >> q)
> >>   		goto error;
> >>
> >>   	for (i = 0; i < priv->num_rx_ring[q]; i++) {
> >> -		skb = netdev_alloc_skb(ndev, info->max_rx_len);
> >> +		skb = __netdev_alloc_skb(ndev, info->max_rx_len, GFP_KERNEL);
> >>   		if (!skb)
> >>   			goto error;
> >>   		ravb_set_buffer_align(skb);
> >> --
> >> 2.32.0
> >
> >

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