Re: >20 KB URBs + EHCI = bad performance due to stalls
From: Maciej S. Szmigiero
Date: Wed Mar 31 2021 - 17:22:45 EST
On 31.03.2021 21:55, Alan Stern wrote:
On Wed, Mar 31, 2021 at 08:20:56PM +0200, Maciej S. Szmigiero wrote:
On 29.03.2021 17:22, Alan Stern wrote:
On Sat, Mar 27, 2021 at 04:55:20PM +0100, Maciej S. Szmigiero wrote:
Hi,
Is there any specific reason that URBs without URB_SHORT_NOT_OK flag that
span multiple EHCI qTDs have Alternate Next qTD pointer set to the dummy
qTD in their every qTD besides the last one (instead of to the first qTD
of the next URB to that endpoint)?
Quick answer: I don't know. I can't think of any good reason. This
code was all written a long time ago. Maybe the issue was overlooked
or the details were misunderstood.
I've dug out the original EHCI driver, that landed in 2.5.2:
https://marc.info/?l=linux-usb-devel&m=100875066109269&w=2
https://marc.info/?l=linux-usb-devel&m=100982880716373&w=2
It already had the following qTD setup code, roughly similar to what
the current one does:
/* previous urb allows short rx? maybe optimize. */
if (!(last_qtd->urb->transfer_flags & USB_DISABLE_SPD)
&& (epnum & 0x10)) {
// only the last QTD for now
last_qtd->hw_alt_next = hw_next;
The "for now" language seems to suggest that ultimately other-than-last
qTDs were supposed to be set not to stall the queue, too.
Just the code to handle this case was never written.
Probably it just slipped out of the developer's mind.
It seems to me though, this should be possible with relatively few
changes to the code:
qh_append_tds() will need to patch these other-than-last qTDs
hw_alt_next pointer to point to the (new) dummy qTD (instead of just
pointing the last submitted qTD hw_next to it and adding the remaining
qTDs verbatim to the qH qTD list).
Right.
Then qh_completions() will need few changes:
*
} else if (IS_SHORT_READ (token)
&& !(qtd->hw_alt_next
& EHCI_LIST_END(ehci))) {
This branch will need to be modified not to mark the queue as stopped
and request its unlinking when such type of short qTD was processed.
This would be a good place to introduce a macro. For example:
} else if (IS_SHORT_READ(token) &&
EHCI_PTR_IS_SET(qtd->hw_alt_next)) {
or something similar.
I agree, this certainly looks more readable.
* The ACTIVE bit should be treated as unset on any qTD following the
one that hits the above condition until a qTD for a different URB is
encountered.
Otherwise the unprocessed remaining qTDs from that short URB will be
considered pending active qTDs and the code will wait forever for their
processing,
The treatment shouldn't be exactly the same as if ACTIVE is clear. The
following qTDs can be removed from the list and deallocated immediately,
since the hardware won't look at them. And they shouldn't affect the
URB's status.
From my understanding of qh_completions() if these "remaining" qTDs from
a short read are treated as !ACTIVE then none of the conditions in
!ACTIVE branch will hit: they don't have DBE or HALT set and they aren't
queue-stopping short read qTDs (I am assuming here that the
aforementioned qtd->hw_alt_next condition will be changed to exclude
them).
So the qTD processing loop will reach "if (last_status == -EINPROGRESS)",
this will be false since previous qTD (that one that has actually
partially completed) had already set the last_status to -EREMOTEIO.
Then the code will delete this qTD from qTD list and go to the next qTD
(either next "remaining" qTD from that URB or from a different URB).
Once a qTD from a different URB is encountered the special treatment of
ACTIVE qTDs as !ACTIVE has to end.
* The code that patches the previous qTD hw_next pointer when removing a
qTD that isn't currently at the qH will need changing to also patch
hw_alt_next pointers of the qTDs belonging to the previous URB in case
the previous URB was one of these short-read-ok ones.
Yes. Awkward but necessary.
Although I know nothing at all about the USB API in Windows, I suspect
that it manages to avoid this awkwardness entirely by not allowing URBs
in the middle of the queue to be unlinked. Or perhaps allowing it only
for endpoint 0. I've often wished Linux's API had been written that
way.
According to Microsoft docs every IRP that might remain queued for an
indefinite interval has to have a cancel handler.
URBs are submitted wrapped in IRPs, so at least in theory it should be
possible to cancel them.
But I don't know how it works in practice.
I also remember getting "warning: guest updated active QH" often when
launching Windows VMs in QEMU.
That does not seem like a good sign, but it might ultimately be a
deficiency in QEMU EHCI HC emulation, not Windows.
In Linux at least we could (theoretically) change the API and modify
all the client drivers.
But I think the benefit is not worth the cost at that point.
That's was my quick assessment what is required to handle these
transactions effectively in the EHCI driver.
I suspect, however, there may be some corner cases involving
non-ordinary qTD unlinking which might need fixing, too (like caused
by usb_unlink_urb(), system suspend or HC removal).
But I am not sure about this since I don't know this code well.
Those shouldn't present any difficulty. There are inherently easier to
handle because the QH won't be actively running when they occur.
I've meant that these can exercise a different code path than ordinary
unlink so one has to check this, too.
(...)
That's what I had on mind by saying that it seems strange to me that
the URB buffer size should be managed by a particular USB device driver
depending on the host controller in use.
In short, the behavior you observed is a bug, resulting in a loss of
throughput (though not in any loss of data). It needs to be fixed.
If you would like to write and submit a patch, that would be great.
Otherwise, I'll try to find time to work on it.
Unfortunately, I doubt I will be able to work on this in coming weeks
due to time constraints, I'm sorry :(
All right, then I'll work on it when time permits.
That's great, thanks.
I would appreciate any effort you could make toward checking the code
in qh_completions(); I suspect that the checks it does involving
EHCI_LIST_END may not be right. At the very least, they should be
encapsulated in a macro so that they are easier to understand.
I've went through the (short) URB linking and unlinking code
(including qh_completions()) and I haven't found anything suspicious
there, besides one thing that's actually on the URB *linking* path:
in qh_append_tds() the dummy qTD that is the last qTD in that
endpoint queue is being overwritten using an assignment operator.
While both this dummy qTD and the source qTD that overwrites it have
the HALT bit set it looks a bit uncomfortable to me to see a qTD that
the HC might just be fetching (while trying to advance the queue) being
overwritten.
I agree. But there's no way around it; if you're going to change the
contents of the qTD queue while the QH is running, at some point you
have to overwrite something that the controller might be accessing
concurrently.
I agree, that's a bit unfortunate.
Unless one unlinks the qH temporarily (but as we have observed that's
rather slow) or disables the async list for a moment (probably slow,
too, and impacts other endpoints).
Like, is C standard giving guarantees that no intermediate values are
being written to a struct when that struct is a target of an assignment
operator?
THe C standard doesn't say anything like that, but the kernel does
generally rely on such behavior.
I see, thanks.
However, it wouldn't hurt to mark this
special case by using WRITE_ONCE.
I think WRITE_ONCE() at least to the hw_token would make a lot of sense
here.
But apparently this doesn't cause trouble, so I guess in practice
this works okay.
Yes, it does.
Alan Stern
Thanks,
Maciej