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Message-ID: <20150306214908.GA15939@electric-eye.fr.zoreil.com>
Date: Fri, 6 Mar 2015 22:49:08 +0100
From: Francois Romieu <romieu@...zoreil.com>
To: Nicolas Schichan <nschichan@...ebox.fr>
Cc: "David S. Miller" <davem@...emloft.net>,
Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mv643xx_eth: only account for work done in
rxq_process in poll callback.
Nicolas Schichan <nschichan@...ebox.fr> :
[...]
> I was trying to minimize the code changes wrt the current source, but if you
> want that change to be in the same patch, I can certainly respin a V2 with it.
No need to respin, you elegantly minimized the changes. The body of the
while loop could be more spartan, especially with bql in sight. Say:
@@ -1041,52 +1041,37 @@ out:
mp->work_tx_end &= ~(1 << txq->index);
}
-static int txq_reclaim(struct tx_queue *txq, int budget, int force)
+static void txq_reclaim(struct tx_queue *txq, int force)
{
struct mv643xx_eth_private *mp = txq_to_mp(txq);
- struct netdev_queue *nq = netdev_get_tx_queue(mp->dev, txq->index);
- int reclaimed;
+ struct net_device *dev = mp->dev;
+ struct netdev_queue *nq = netdev_get_tx_queue(dev, txq->index);
+ int used = txq->tx_used_desc;
+ int count;
- __netif_tx_lock_bh(nq);
+ mp->work_tx &= ~(1 << txq->index);
- reclaimed = 0;
- while (reclaimed < budget && txq->tx_desc_count > 0) {
- int tx_index;
- struct tx_desc *desc;
- u32 cmd_sts;
- char desc_dma_map;
-
- tx_index = txq->tx_used_desc;
- desc = &txq->tx_desc_area[tx_index];
- desc_dma_map = txq->tx_desc_mapping[tx_index];
+ __netif_tx_lock(nq, smp_processor_id());
- cmd_sts = desc->cmd_sts;
+ for (count = txq->tx_desc_count; count > 0; count--) {
+ struct tx_desc *desc = &txq->tx_desc_area[used];
+ char desc_dma_map = txq->tx_desc_mapping[used];
- if (cmd_sts & BUFFER_OWNED_BY_DMA) {
+ if (desc->cmd_sts & BUFFER_OWNED_BY_DMA) {
if (!force)
break;
- desc->cmd_sts = cmd_sts & ~BUFFER_OWNED_BY_DMA;
+ desc->cmd_sts &= ~BUFFER_OWNED_BY_DMA;
}
- txq->tx_used_desc = tx_index + 1;
- if (txq->tx_used_desc == txq->tx_ring_size)
- txq->tx_used_desc = 0;
-
- reclaimed++;
- txq->tx_desc_count--;
+ used = (used + 1) % txq->tx_ring_size;
if (!IS_TSO_HEADER(txq, desc->buf_ptr)) {
-
if (desc_dma_map == DESC_DMA_MAP_PAGE)
- dma_unmap_page(mp->dev->dev.parent,
- desc->buf_ptr,
- desc->byte_cnt,
- DMA_TO_DEVICE);
+ dma_unmap_page(dev->dev.parent, desc->buf_ptr,
+ desc->byte_cnt, DMA_TO_DEVICE);
else
- dma_unmap_single(mp->dev->dev.parent,
- desc->buf_ptr,
- desc->byte_cnt,
- DMA_TO_DEVICE);
+ dma_unmap_single(dev->dev.parent, desc->buf_ptr,
+ desc->byte_cnt, DMA_TO_DEVICE);
}
if (cmd_sts & TX_ENABLE_INTERRUPT) {
@@ -1097,18 +1082,15 @@ static int txq_reclaim(struct tx_queue *txq, int budget, int force)
}
if (cmd_sts & ERROR_SUMMARY) {
- netdev_info(mp->dev, "tx error\n");
- mp->dev->stats.tx_errors++;
+ netdev_info(dev, "tx error\n");
+ dev->stats.tx_errors++;
}
-
}
- __netif_tx_unlock_bh(nq);
-
- if (reclaimed < budget)
- mp->work_tx &= ~(1 << txq->index);
+ txq->tx_used_desc = used;
+ txq->tx_desc_count = count;
- return reclaimed;
+ __netif_tx_unlock(nq);
}
Nobody uses txq_reclaim return status code. You can turn it void.
[...]
> > work_tx is also updated in irq context. I'd rather see "clear_flag() then
> > reclaim()" than "reclaim() then clear_flag()" in a subsequent patch.
>
> Just to be sure that I understand the issue here, under normal conditions,
> work_tx is updated in irq context via mv643xx_eth_collect_events() and then
> the mv643xx interrupts are masked and napi_schedule() is called. Only once all
> the work has been completed in the poll callback and the work flags have been
> cleared, are the interrupt unmasked and napi_complete() is called. As far as I
> can see there should be no issue here.
IRQF_SHARED and mv643xx_eth_netpoll make me marginally nervous.
My concern was mostly stylistic given the current code. Things get
easier - call me lazy - if the bit is cleared before reclaiming.
> The only problem I can see is in OOM condition when napi_schedule is called
> from a timer callback (oom_timer_wrapper()) which will result in the poll
> callback being called with the interrupts unmasked and if an interrupt fires
> (possibly in an other CPU) at the wrong time, mv643xx_eth_collect_events()
> will race with the various flags clear in txq_reclaim, rxq_process and
> rxq_refill ?
<knee jerk>
You can ignore OOM: the driver should drop received packets immediately
instead of poking holes in its receive descriptor ring.
</knee jerk>
> In that case wouldn't be something like clear_bit/set_bit preferred compared
> to the direct bitwise operations ?
I don't have any strong feeling about it.
Thanks.
--
Ueimor
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