Am 19.10.2016 um 09:57 hat Dmitry Fleytman geschrieben: > > 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.
For me everything hints at this being only an example: Not only do the numbers match the example made in the previous sentence (which is explicitly called an example) and look weird as a real limit, but it's also in the same paragraph as the explicit example and the spec is generally good at starting a new paragraph when talking about a new aspect. I don't care enough to actually make you change anything, but I wanted you to be aware that the interpretation of the spec as coded into our emulation isn't clear at all (in fact, I would think it's clear that it's _not_ meant this way) and that real hardware probably doesn't do the same thing as we do. What we're doing may still have merit, as a workaround for a guest driver bug. > 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. Commit 9596ef7c was good in flagging it as a guest driver bug. Only a later series brought in the questionable spec interpretation. Kevin
