linux-kernel - Re: [PATCH] ravb: Use GFP_KERNEL instead of GFP

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Date:   Sun, 20 Feb 2022 09:48:52 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
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" <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

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://elixir.bootlin.com/linux/v5.17-rc4/source/include/linux/skbuff.h#L2945

[2]: 
https://elixir.bootlin.com/linux/v5.17-rc4/source/drivers/net/ethernet/renesas/ravb_main.c#L470

[3]: 
https://elixir.bootlin.com/linux/v5.17-rc3/source/net/core/skbuff.c#L488


> 
> [1] https://www.kernel.org/doc/htmldocs/networking/API-netdev-alloc-skb.html
> [2] https://www.kernel.org/doc/htmldocs/networking/API---netdev-alloc-skb.html
> 
> 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
> 
>