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

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Date:	Thu, 4 Dec 2014 12:20:13 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...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 Thu, Dec 04, 2014 at 03:03:34AM +0008, Jason Wang wrote:
> 
> 
> 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.

I refer to this field:
        struct tun_file __rcu   *tfiles[MAX_TAP_QUEUES];
if we make MAX_TAP_QUEUES 256, this will use 4K bytes,
apparently unconditionally.


> 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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