On 3/5/06, Andrew Morton <[email protected]> wrote:
> Chris Leech <[email protected]> wrote:
> >
> > +#ifdef CONFIG_NET_DMA
> > + tp->ucopy.dma_chan = NULL;
> > + if ((len > sysctl_tcp_dma_copybreak) && !(flags & MSG_PEEK) && !sysctl_tcp_low_latency && __get_cpu_var(softnet_data.net_dma))
> > + dma_lock_iovec_pages(msg->msg_iov, len, &tp->ucopy.locked_list);
> > +#endif
>
> The __get_cpu_var() here will run smp_processor_id() from preemptible
> context. You'll get a big warning if the correct debug options are set.
>
> The reason for this is that preemption could cause this code to hop between
> CPUs.
I've been playing with different models of where to select which DMA
channel to use in order to reduce cache thrash and lock contention in
the driver. It's not a clean per-cpu issue because per I/O there are
potentially operations happening in both the process syscall and the
netrx softirq context.
Right now the code delays selection of a DMA channel until the first
offload copy is ready to go, so the __get_cpu_var() you point out is
just checking to see if any hardware exists for I/OAT at this point
before doing the page pinning. Before anything is done with the
channel the per-cpu pointer is re-read safely with preemption disabled
and a reference count is incremented.
- Chris
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