On Fri, Jan 23, 2015 at 12:33 PM, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote: > On Fri, 2015-01-23 at 10:18 +0000, Grumbach, Emmanuel wrote: > >> I don't think it will introduce that much of latency. >> Note that there is a call to flush() right after that, this means that >> any driver which implements this call needs to wait there. So you have >> the latency in either place. The only additional latency it adds is for >> other RCU sections / packets on other interfaces. > > This is correct. > >> Also - since we just stopped the netif, there can possibly be only one >> packet for each vif / ac processing. This is not too much data to >> process. > > But this is the wrong conclusion. > > synchronize_rcu() is expensive, no matter what, especially if you have > multiple cores. The RCU grace periods will practically never line up, > and it needs to wait for every CPU to go through one. > I know. > It's not actually just "one packet for each vif/ac" - it's a whole tail > of whatever other RCU usages there are. Back when this was discussed, > the wizery people measured this to hundreds of ms in many not too > uncommon cases. > That's the part I didn't know. I wasn't aware that this synchronize_net() was there and that someone removed it. >> Your call, but to honest, I have been optimizing the roaming as well (as >> you know). And it was really bad for drivers that implements the flush() >> callback. Especially in cases where you are already far from the AP you >> want to roam from. This is why I added the drop parameter and other >> optimizations (don't flush() after probing etc...) > > Well, yes, but that's an upper bound - here with the synchronize_net() > we're more talking about a lower bound. > I also have to admit I was wrong earlier: the flush() call in set_disassoc has drop set to true, so this call will return immediately. I just wonder how we handle the case where we still have packets in the Tx path and we bring everything down. I can check the code, but not right now. -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
