Re: iwlagn: order 2 page allocation failures

Mel Gorman
2009-09-11T08:45:40Z

On Thu, Sep 10, 2009 at 11:15:47AM -0700, reinette chatre wrote:
> > > We can thus use ___GFP_NOWARN for the allocations in
> > > iwl_rx_allocate and leave it to the restocking to find the needed memory
> > > when it tried its allocations using GFP_KERNEL.
> > >
> >
> > Would it be possible to use __GFP_NOWARN *unless* this allocation is
> > necessary to receive the packet?
>
> I think so.
>
> > > I do think it is useful to let user know about these allocation
> > > failures, even if it does not result in packet loss. Wrapping it in
> > > net_ratelimit() will help though.
> > >
> >
> > If it does not distinguish between failures causing packet loss and just a
> > temporary issue, I'd be worried the messages would generate bug reports and
> > we genuinely won't know if it's a real problem or not.
>
> Good point.
>
> >
> > As a total aside, there is still the problem that the driver is depending on
> > order-2 allocations. On systems without swap, the allocation problem could be
> > more severe as there are fewer pages the system can use to regain contiguity.
>
> It seems that somebody did think about this in the initialization of
> max_pkt_size (which is priv->hw_params.rx_buf_size - 256). If we use
> max_pkt_size in the code that allocates the skb then the 256 added for
> alignment will not cause it to go to an order-2 allocation. I do not
> know why max_pkt_size is not used at the moment and will have to do some
> digging to find out.
>

Thanks

> > > How about the patch below?
> > >
> > > diff --git a/drivers/net/wireless/iwlwifi/iwl-rx.c b/drivers/net/wireless/iwlwifi/iwl-rx.c
> > > index b90adcb..f0ee72e 100644
> > > --- a/drivers/net/wireless/iwlwifi/iwl-rx.c
> > > +++ b/drivers/net/wireless/iwlwifi/iwl-rx.c
> > > @@ -252,10 +252,11 @@ void iwl_rx_allocate(struct iwl_priv *priv, gfp_t priority)
> > >
> > > /* Alloc a new receive buffer */
> > > skb = alloc_skb(priv->hw_params.rx_buf_size + 256,
> > > - priority);
> > > + priority | __GFP_NOWARN);
> > >
> >
> > So, would it be possible here to only remove __GFP_NOWARN if this is GFP_ATOMIC
> > (implying we have to refill now) and the rxq->free_count is really small
> > e.g. <= 2?
>
> I like your suggestion. Considering the issue I would like to remove
> __GFP_NOWARN even if it is GFP_KERNEL ... I think it is actually even
> more of a problem if we are in GFP_KERNEL and not able to allocate
> memory when running low on buffers. Also, with the queue size of 256 I
> think we can use RX_LOW_WATERMARK here, which is 8.
>

RX_LOW_WATERMARK sounds reasonable as if that watermark is reached, the
buffer count is pretty low. With order-2 allocations, I bet the system is
beginning to grind a bit to find contiguous pages at that point as well.

I agree that it's a greater problem if the system is unable to allocate
the pages as GFP_KERNEL - prehaps to the extent where it's worth
distinguishing between GFP_KERNEL and GFP_ATOMIC failures. If GFP_KERNEL
allocations are failure, packet loss is likely and the system may not
recover, particularly if there is no swap configured.

> >
>
> >
> > > if (!skb) {
> > > - IWL_CRIT(priv, "Can not allocate SKB buffers\n");
> > > + if (net_ratelimit())
> > > + IWL_CRIT(priv, "Can not allocate SKB buffer.\n");
> >
> > Similarly, could the message either be supressed when there is enough
> > buffers in the RX queue or differenciate between enough buffers and
> > things getting tight possibly causing packet loss?
>
> Frans also had comments in this regard. Will do.
>
> >
> > The IWL_CRIT() part even is a hint - there is no guarantee that the allocation
> > failure is really a critical problem.
>
> Right.
>
> How about this:
>
> >From bd2153dd9e4a0ad588adec38c580d67023d5587e Mon Sep 17 00:00:00 2001
> From: Reinette Chatre <reinette.chatre@intel.com>
> Date: Wed, 9 Sep 2009 15:41:00 -0700
> Subject: [PATCH] iwlwifi: reduce noise when skb allocation fails
>
> Replenishment of receive buffers is done in the tasklet handling
> received frames as well as in a workqueue. When we are in the tasklet
> we cannot sleep and thus attempt atomic skb allocations. It is generally
> not a big problem if this fails since iwl_rx_allocate is always followed
> by a call to iwl_rx_queue_restock which will queue the work to replenish
> the buffers at a time when sleeping is allowed.
>
> We thus add the __GFP_NOWARN to the skb allocation in iwl_rx_allocate to
> reduce the noise if such an allocation fails while we still have enough
> buffers. We do maintain the warning and the error message when we are low
> on buffers to communicate to the user that there is a potential problem with
> memory availability on system
>
> This addresses issue reported upstream in thread "iwlagn: order 2 page
> allocation failures" in
> http://thread.gmane.org/gmane.linux.kernel.wireless.general/39187
>
> Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
> ---
> drivers/net/wireless/iwlwifi/iwl-rx.c | 12 +++++++++---
> drivers/net/wireless/iwlwifi/iwl3945-base.c | 8 +++++++-
> 2 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/wireless/iwlwifi/iwl-rx.c b/drivers/net/wireless/iwlwifi/iwl-rx.c
> index b90adcb..cb50cc7 100644
> --- a/drivers/net/wireless/iwlwifi/iwl-rx.c
> +++ b/drivers/net/wireless/iwlwifi/iwl-rx.c
> @@ -250,12 +250,18 @@ void iwl_rx_allocate(struct iwl_priv *priv, gfp_t priority)
> }
> spin_unlock_irqrestore(&rxq->lock, flags);
>
> + if (rxq->free_count > RX_LOW_WATERMARK)
> + priority |= __GFP_NOWARN;

Seems very reasonable.

> /* Alloc a new receive buffer */
> - skb = alloc_skb(priv->hw_params.rx_buf_size + 256,
> - priority);
> + skb = alloc_skb(priv->hw_params.rx_buf_size + 256, priority);
>

This change appears superflous. It don't change any functionality. Looks
like the style is just being made consistent with a similar code block
elsewhere.

> if (!skb) {
> - IWL_CRIT(priv, "Can not allocate SKB buffers\n");
> + if (net_ratelimit())
> + IWL_DEBUG_INFO("Failed to allocate SKB buffer.\n");
> + if ((rxq->free_count <= RX_LOW_WATERMARK) &&
> + net_ratelimit())
> + IWL_CRIT(priv, "Failed to allocate SKB buffer. Only %u free buffers remaining\n",
> + rxq->free_count);

To get a good idea of how screwed we really are, how about?

IWL_CRIT(priv, "Failed to allocate SKB buffer with %s. Only %u free buffers remaining\n",
priority == GFP_ATOMIC ? "GFP_ATOMIC" : "GFP_KERNEL",
rxq->free_count);

> /* We don't reschedule replenish work here -- we will
> * call the restock method and if it still needs
> * more buffers it will schedule replenish */
> diff --git a/drivers/net/wireless/iwlwifi/iwl3945-base.c b/drivers/net/wireless/iwlwifi/iwl3945-base.c
> index 0909668..0d96b17 100644
> --- a/drivers/net/wireless/iwlwifi/iwl3945-base.c
> +++ b/drivers/net/wireless/iwlwifi/iwl3945-base.c
> @@ -1146,11 +1146,17 @@ static void iwl3945_rx_allocate(struct iwl_priv *priv, gfp_t priority)
> }
> spin_unlock_irqrestore(&rxq->lock, flags);
>
> + if (rxq->free_count > RX_LOW_WATERMARK)
> + priority |= __GFP_NOWARN;
> /* Alloc a new receive buffer */
> skb = alloc_skb(priv->hw_params.rx_buf_size, priority);
> if (!skb) {
> if (net_ratelimit())
> - IWL_CRIT(priv, ": Can not allocate SKB buffers\n");
> + IWL_DEBUG_INFO("Failed to allocate SKB buffer.\n");
> + if ((rxq->free_count <= RX_LOW_WATERMARK) &&
> + net_ratelimit())
> + IWL_CRIT(priv, "Failed to allocate SKB buffer. Only %u free buffers remaining\n",
> + rxq->free_count);
> /* We don't reschedule replenish work here -- we will
> * call the restock method and if it still needs
> * more buffers it will schedule replenish */

Otherwise, it looks just the finest and I think it will address the
problem to some extent - in that it won't print alarming messages when
they are not needed.

The additional changes with respect to GFP_ATOMIC are optional. Whether
you do it or not.

Acked-by: Mel Gorman <mel@csn.ul.ie>

Thanks very much.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab