linux-kernel - Re: [PATCH v3 net-next 2/2 tuntap: Increase the number of queues in tun.

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Message-Id: <1417661734.16500.0@smtp.corp.redhat.com>
Date:	Thu, 04 Dec 2014 03:03:34 +0008
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Pankaj Gupta <pagupta@...hat.com>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, davem@...emloft.net, dgibson@...hat.com,
	vfalico@...il.com, edumazet@...gle.com, vyasevic@...hat.com,
	hkchu@...gle.com, wuzhy@...ux.vnet.ibm.com, xemul@...allels.com,
	therbert@...gle.com, bhutchings@...arflare.com, xii@...gle.com,
	stephen@...workplumber.org, jiri@...nulli.us,
	sergei.shtylyov@...entembedded.com
Subject: Re: [PATCH v3 net-next 2/2 tuntap: Increase the number of queues in
 tun.



On Wed, Dec 3, 2014 at 5:52 PM, Michael S. Tsirkin <mst@...hat.com> 
wrote:
> On Wed, Dec 03, 2014 at 12:49:37PM +0530, Pankaj Gupta wrote:
>>  Networking under kvm works best if we allocate a per-vCPU RX and TX
>>  queue in a virtual NIC. This requires a per-vCPU queue on the host 
>> side.
>>  
>>  It is now safe to increase the maximum number of queues.
>>  Preceding patche: 'net: allow large number of rx queues'
> 
> s/patche/patch/
> 
>>  made sure this won't cause failures due to high order memory
>>  allocations. Increase it to 256: this is the max number of vCPUs
>>  KVM supports.
>>  
>>  Signed-off-by: Pankaj Gupta <pagupta@...hat.com>
>>  Reviewed-by: David Gibson <dgibson@...hat.com>
> 
> Hmm it's kind of nasty that each tun device is now using x16 memory.
> Maybe we should look at using a flex array instead, and removing the
> limitation altogether (e.g. make it INT_MAX)?

But this only happens when IFF_MULTIQUEUE were used.
And core has vmalloc() fallback.
So probably not a big issue?

> 
> 
> 
>>  ---
>>   drivers/net/tun.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>  
>>  diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>  index e3fa65a..a19dc5f8 100644
>>  --- a/drivers/net/tun.c
>>  +++ b/drivers/net/tun.c
>>  @@ -113,10 +113,11 @@ struct tap_filter {
>>   	unsigned char	addr[FLT_EXACT_COUNT][ETH_ALEN];
>>   };
>>   
>>  -/* DEFAULT_MAX_NUM_RSS_QUEUES were chosen to let the rx/tx queues 
>> allocated for
>>  - * the netdevice to be fit in one page. So we can make sure the 
>> success of
>>  - * memory allocation. TODO: increase the limit. */
>>  -#define MAX_TAP_QUEUES DEFAULT_MAX_NUM_RSS_QUEUES
>>  +/* MAX_TAP_QUEUES 256 is chosen to allow rx/tx queues to be equal
>>  + * to max number of vCPUS in guest. Also, we are making sure here
>>  + * queue memory allocation do not fail.
> 
> It's not queue memory allocation anymore, is it?
> I would say "
> This also helps the tfiles field fit in 4K, so the whole tun
> device only needs an order-1 allocation.
> "
> 
>>  + */
>>  +#define MAX_TAP_QUEUES 256
>>   #define MAX_TAP_FLOWS  4096
>>   
>>   #define TUN_FLOW_EXPIRE (3 * HZ)
>>  -- 
>>  1.8.3.1
>>  
>>  --
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