Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
On Sun, Feb 10, 2008 at 11:26:06AM +, Paul Brook wrote: On Sunday 10 February 2008, Blue Swirl wrote: On 2/9/08, H. Peter Anvin [EMAIL PROTECTED] wrote: Blue Swirl wrote: If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns. Thanks, this clarified the difference. But I'll rephrase my original comment: The patch looks OK, but the simulated FIFO exhaustion should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html The difference is you *can't* do that in a general layer. What makes you think that is impossible? IIUC the proposed patch makes the serial driver return an empty FIFO exactly once, them immediately continue receiving data. Throughput should be approximately the same, you've just got a bit of extra overhead to process the additional interrupts. This is very different to the previous patch which did time-based throughput limiting. You can't do this in generic code because there's no way to guess when the guest os has seen the FIFO empty condition. The best you can do is pause for some arbitrary length of time, which is both unreliable (the guest OS may not have got to far enough yet, especially if the host machine is heavily loaded), and has a significant negative impact on throughput. Also win2k install hack in ide.c seems to be related to this problem, so even more generic solution would be desirable. IIUC the win2k hack is an actual timing problem. The win2k IDE drivers are buggy, and fall over if the drive responds too soon. Is everybody convinced about this patch now? I would really like to see it in the CVS, as it greatly improves the usability of the serial consoles on targets running GNU/Linux (and probably OSes). -- .''`. Aurelien Jarno | GPG: 1024D/F1BCDB73 : :' : Debian developer | Electrical Engineer `. `' [EMAIL PROTECTED] | [EMAIL PROTECTED] `-people.debian.org/~aurel32 | www.aurel32.net
Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
On 2/9/08, H. Peter Anvin [EMAIL PROTECTED] wrote: Blue Swirl wrote: If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns. Thanks, this clarified the difference. But I'll rephrase my original comment: The patch looks OK, but the simulated FIFO exhaustion should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html The difference is you *can't* do that in a general layer. What makes you think that is impossible? Just move the serial_clear_burst to vl.c under name chr_clear_burst, move burst_len to CharDriverState and introduce a new function in vl.c that contains the burst length check. This is functionally identical to your patch. For 100% compatibility, the init functions could be changed so that only PC serial is affected, but I think all character devices would benefit from this. Also win2k install hack in ide.c seems to be related to this problem, so even more generic solution would be desirable.
Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
On Sunday 10 February 2008, Blue Swirl wrote: On 2/9/08, H. Peter Anvin [EMAIL PROTECTED] wrote: Blue Swirl wrote: If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns. Thanks, this clarified the difference. But I'll rephrase my original comment: The patch looks OK, but the simulated FIFO exhaustion should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html The difference is you *can't* do that in a general layer. What makes you think that is impossible? IIUC the proposed patch makes the serial driver return an empty FIFO exactly once, them immediately continue receiving data. Throughput should be approximately the same, you've just got a bit of extra overhead to process the additional interrupts. This is very different to the previous patch which did time-based throughput limiting. You can't do this in generic code because there's no way to guess when the guest os has seen the FIFO empty condition. The best you can do is pause for some arbitrary length of time, which is both unreliable (the guest OS may not have got to far enough yet, especially if the host machine is heavily loaded), and has a significant negative impact on throughput. Also win2k install hack in ide.c seems to be related to this problem, so even more generic solution would be desirable. IIUC the win2k hack is an actual timing problem. The win2k IDE drivers are buggy, and fall over if the drive responds too soon. Paul
Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
On 2/9/08, H. Peter Anvin [EMAIL PROTECTED] wrote: Blue Swirl wrote: On 2/9/08, Rob Landley [EMAIL PROTECTED] wrote: Here's a patch Peter Anvin wrote so the serial I/O doesn't flood the kernel. The patch looks OK, but the throttling should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html I strongly disagree with the sentiments in that post. This is not a matter of rate throttling, but simulated FIFO exhaustion -- they are NOT the same thing. Simulated FIFO exhaustion is functionally equivalent to making sure there are interrupt windows opened in an otherwise-too-long critical section; it doesn't constrain any particular flow rate, as it still permits another interrupt to immediately come in. If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns. Thanks, this clarified the difference. But I'll rephrase my original comment: The patch looks OK, but the simulated FIFO exhaustion should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html
Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
Blue Swirl wrote: If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns. Thanks, this clarified the difference. But I'll rephrase my original comment: The patch looks OK, but the simulated FIFO exhaustion should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html The difference is you *can't* do that in a general layer. -hpa
[Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
Here's a patch Peter Anvin wrote so the serial I/O doesn't flood the kernel.
Here's the thread on linux-kernel aboout it:
http://lkml.org/lkml/2008/2/5/401
Rob
On Thursday 07 February 2008 15:19:39 you wrote:
Rob Landley wrote:
Specifically, qemu isn't paravirtualized, it's fully virtualized. The
same kernel can run on real hardware just fine. (Sort of the point of
the project...)
I can yank the warning for the kernels I build (or set PASS_LIMIT to
999), but I'd rather not carry any more patches than I can avoid...
Patch attached, completely untested beyond compilation.
In particular:
- we should probably clear the burst counter when the interrupt
line goes from inactive to active.
- there probably should be a timer which clears the burst
counter.
However, I think I've covered most of the bases...
-hpa
diff --git a/hw/serial.c b/hw/serial.c
index b1bd0ff..c902792 100644
--- a/hw/serial.c
+++ b/hw/serial.c
@@ -73,6 +73,15 @@
#define UART_LSR_OE0x02/* Overrun error indicator */
#define UART_LSR_DR0x01/* Receiver data ready */
+/*
+ * It's common for an IRQ handler to keep reading the RBR until
+ * the LSR indicates that the FIFO is empty, expecting that the
+ * CPU is vastly faster than the serial line. This can cause
+ * overruns or error indications if the FIFO never empties, so
+ * give the target OS a breather every so often.
+ */
+#define MAX_BURST 512
+
struct SerialState {
uint16_t divider;
uint8_t rbr; /* receive register */
@@ -91,8 +100,14 @@ struct SerialState {
int last_break_enable;
target_phys_addr_t base;
int it_shift;
+int burst_len;
};
+static void serial_clear_burst(SerialState *s)
+{
+s-burst_len = 0;
+}
+
static void serial_update_irq(SerialState *s)
{
if ((s-lsr UART_LSR_DR) (s-ier UART_IER_RDI)) {
@@ -114,6 +129,8 @@ static void serial_update_parameters(SerialState *s)
int speed, parity, data_bits, stop_bits;
QEMUSerialSetParams ssp;
+serial_clear_burst(s);
+
if (s-lcr 0x08) {
if (s-lcr 0x10)
parity = 'E';
@@ -221,9 +238,12 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t
addr)
ret = s-divider 0xff;
} else {
ret = s-rbr;
-s-lsr = ~(UART_LSR_DR | UART_LSR_BI);
-serial_update_irq(s);
-qemu_chr_accept_input(s-chr);
+ if (s-burst_len MAX_BURST) {
+ s-burst_len++;
+ s-lsr = ~(UART_LSR_DR | UART_LSR_BI);
+ serial_update_irq(s);
+ qemu_chr_accept_input(s-chr);
+ }
}
break;
case 1:
@@ -235,6 +255,7 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t
addr)
break;
case 2:
ret = s-iir;
+ serial_clear_burst(s);
/* reset THR pending bit */
if ((ret 0x7) == UART_IIR_THRI)
s-thr_ipending = 0;
@@ -248,6 +269,10 @@ static uint32_t serial_ioport_read(void *opaque, uint32_t
addr)
break;
case 5:
ret = s-lsr;
+ if (s-burst_len = MAX_BURST)
+ ret = ~(UART_LSR_DR|UART_LSR_BI);
+ if (!(ret UART_LSR_DR))
+ serial_clear_burst(s);
break;
case 6:
if (s-mcr UART_MCR_LOOP) {
--
One of my most productive days was throwing away 1000 lines of code.
- Ken Thompson.
Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
On 2/9/08, Rob Landley [EMAIL PROTECTED] wrote: Here's a patch Peter Anvin wrote so the serial I/O doesn't flood the kernel. The patch looks OK, but the throttling should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html
Re: [Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
Blue Swirl wrote: On 2/9/08, Rob Landley [EMAIL PROTECTED] wrote: Here's a patch Peter Anvin wrote so the serial I/O doesn't flood the kernel. The patch looks OK, but the throttling should benefit all devices, as discussed here: http://lists.gnu.org/archive/html/qemu-devel/2007-12/msg00283.html I strongly disagree with the sentiments in that post. This is not a matter of rate throttling, but simulated FIFO exhaustion -- they are NOT the same thing. Simulated FIFO exhaustion is functionally equivalent to making sure there are interrupt windows opened in an otherwise-too-long critical section; it doesn't constrain any particular flow rate, as it still permits another interrupt to immediately come in. If you look at the patch, there are no timing dependencies; the only parameter is the depth of the virtual queue. The exhaustion is completely controlled by target OS access patterns. -hpa
[Qemu-devel] Re: 2.6.24 says serial8250: too much work for irq4 a lot.
On Tuesday 05 February 2008 15:07:27 H. Peter Anvin wrote: Rob Landley wrote: When running a 2.6.24 kernel built for x86-64 under qemu via serial console, doing CPU-intensive things that also produce a lot of output (such as compiling software) tends to produce the error message in the title. Anybody have a clue why? It doesn't seem to cause an actual problem, but it's kind of annoying. (If it's a qemu issue, I can go bother them. It's possible that qemu isn't delivering interrupts as often as it expects, since that's limited by the granularity of the host timer; I know the clock in qemu can run a bit slow because it only gets clock interrupts when the host system isn't too busy to schedule the emulator. But this doesn't usually cause a problem. I _think_ the message is just a this should never happen type warning, which is happening to me. But I break stuff. :) This is because Qemu spews data to the serial port without any rate limiting; this causes the in-kernel serial port driver to think the port is stuck. The serial port emulation needs to make it possible to drain the virtual FIFO every now and then, as opposed to filling it again immediately. -hpa Thanks. cc'd the right list on this one. :) Rob -- One of my most productive days was throwing away 1000 lines of code. - Ken Thompson.
