> On 19 Oct 2016, at 10:25 AM, Kevin Wolf <kw...@redhat.com> wrote: > > Am 19.10.2016 um 08:48 hat Dmitry Fleytman geschrieben: >> Another related thing that I noticed while debugging this and turning on >> tracing is that the interrupt throttling timers kept firing even if >> there was no activity at all. Something might be wrong, there, too. >> >> Next thing I wondered why throttling was enabled at all because the spec >> says the default is 0 (turned off). So one thing that I'm pretty sure is >> just a misunderstanding is the following defintion: >> >> #define E1000E_MIN_XITR (500) /* No more then 7813 interrupts per >> second according to spec 10.2.4.2 */ >> >> >> As I understand it, the spec is just giving an example there and lower >> values are valid as well. At the very least, 0 should be accepted as a >> special case because it means "disabled" and it's specified to be the >> default. >> >> >> Right, this according to the spec this value should be 0 by default and >> throttling should be disabled. >> >> Current device implementation does not allow specification of >> throttling interval less than 500 and treats interval 0 as throttling >> enabled with interval 500. >> >> This is done by intention because according to the spec (10.2.4.2) >> device cannot produce more than 7813 interrupts per second even when >> throttling is disabled. Therefore, even in case of interrupt storm >> (continuous interrupt re-injection by device), number of interrupts >> produced by device is limited and CPU (driver) has enough time to do >> its job and handle problematic interrupt state. > > I think you're misinterpreting the spec here. This is the paragraph > we're talking about, right? > > For example, if the interval is programmed to 500 (decimal), the > 82574 guarantees the CPU is not interrupted by it for 128 µs from > the last interrupt. The maximum observable interrupt rate from the > 82574 should never exceed 7813 interrupts/sec. > > It says "for example", so this is just demonstrating how you can > calculate the effects of a specific throttling setting. It says that > _if_ you set ITR to 500, you get an interrupt at most every > 500 * 256 ns = 128 µs. And 1 / 128 µs = 7821.5 Hz, so this is the > effective maximum frequency that _this specific_ ITR setting allows. > > I also don't think it would make any sense for hardware to be unable to > trigger interrupts more often than that. Triggering an interrupt is not > a complex operation that involves a lot of calculation or anything.
Hi Kevin, Yes, I assume that sentence “The maximum observable interrupt rate from the 82574 should never exceed 7813 interrupts/sec." is not a related to a specific case, but describes a generic limitation, however it might be I’m misreading the spec indeed. > >> Opposed to this, virtual device is able to raise interrupts with rate >> limited by CPU speed only therefore driver has no chance to fix >> interrupt storm condition. >> >> Windows e1000e drivers rely on upper limit for number of interrupts >> per second in some cases and absence of this limit leads to infinite >> interrupt storms. >> >> To summarise, while usage of throttling mechanisms is a little bit >> different from what specification says, effective emulated device >> behavior is totally compliant to the real device. > > So Windows doesn't configure ITR (i.e. it is 0) even though it can't > handle unlimited interrupts? That would be a driver bug then, and > perhaps an important enough one to keep a workaround in our code. But > then let's be explicit that this is a workaround for a Windows bug and > not mandated by the spec. > > I'm not sure in what setup you produced this error, but possibly a > reason why this doesn't happen with real hardware isn't the NIC itself > but the backend: Communication with the host can obviously be faster > than talking to a physical network (so if you were doing the latter, the > rate in the VM wouldn't be limited by the CPU, but by the physical > network). This issue is reproduced on device disable and not related to intensive device/backend communication. One RX packet with right timing is enough to trigged the problem. The same issue was fixed in e1000 device some time ago as well. ~Dmitry > >> As this wasn't blocking me after I had patched the above constant >> locally to 0 so that I could see the actually meaningful trace events, I >> didn't dig deeper, but I suspect that my local workaround for the >> trace point spam may actually be a valid fix. >> >> >> This part that looks suspicious. >> >> Even when throttling is enabled, timers should be idle >> unless there are packets on corresponding path. >> >> In case you see times firing when device is idle - something >> might be wrong in throttling mechanisms. > > I saw a lot of trace events related to the timer between the really > interesting information (that is, multiple timer invocations with > nothing else in between), but I think it may not have been always. I > didn't really look too closely at the cause of this one because it was > easy enough to patch it out of the way. > > Kevin
