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 - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
