On Jan 31, 2010, at 2:24 AM, Keir Fraser wrote:
> On 31/01/2010 00:40, "Kaushik Kumar Ram" <kaushik@xxxxxxxx> wrote:
>
>>> So how about making the clear of l1i in the l1 mask unconditional? I think
>>> that would be better, but I wasn't sure it is safe, since the first l1i you
>>> scan you may start halfway through, and thus legitimately have more work to
>>> do on that l1i on a later iteration of the outer loop. But I think that is
>>> the only case it is good to leave the l1 unmasked? Also, even better, on
>>> that second scan of that l1i, you would preferably want to scan only those
>>> bits in the l2 mask that you didn't scan on the first iteration of the outer
>>> loop!
>>
>> OK. I agree the following is a good compromise.
>> - Unconditionally clear l1 bits except the first l1i (but only if l2 is
>> scanned from halfway).
>> - Remember where the scanning began (both l1i and l2i) and stop scanning at
>> that point after wrapping around.
>> - Read active_evtchns() once per l1i (except the first l1i where you might
>> have to do it twice).
>
> Yes, sounds good. Are you going to make the patch?
Here is a first version of the patch. It was more complicated than I expected
and it was also hard for me to decide if it was "efficient" enough.
I can work on improving it, based on your feedback.
# HG changeset patch
# User kaushik@xxxxxxxxxxxxxxxx
# Date 1264994160 21600
# Branch evtchn2
# Node ID e539a849a3348c3e87e8d50eba5998b7cdb9394d
# Parent c88a02a22a057a632e6c21442e42e56e07904988
Fair processing of pending upcalls.
Signed-off-by: Kaushik Kumar Ram <kaushik@xxxxxxxx>
diff -r c88a02a22a05 -r e539a849a334 drivers/xen/core/evtchn.c
--- a/drivers/xen/core/evtchn.c Fri Jan 29 07:57:48 2010 +0000
+++ b/drivers/xen/core/evtchn.c Sun Jan 31 21:16:00 2010 -0600
@@ -236,9 +236,9 @@ static DEFINE_PER_CPU(unsigned int, curr
/* NB. Interrupts are disabled on entry. */
asmlinkage void evtchn_do_upcall(struct pt_regs *regs)
{
- unsigned long l1, l2;
+ unsigned long l1, l2, l2_start = 0;
unsigned long masked_l1, masked_l2;
- unsigned int l1i, l2i, port, count;
+ unsigned int l1i, l2i = 0, port, count, l1i_start, l2i_start;
int irq;
unsigned int cpu = smp_processor_id();
shared_info_t *s = HYPERVISOR_shared_info;
@@ -261,48 +261,96 @@ asmlinkage void evtchn_do_upcall(struct
#endif
l1 = xchg(&vcpu_info->evtchn_pending_sel, 0);
- l1i = per_cpu(current_l1i, cpu);
- l2i = per_cpu(current_l2i, cpu);
+ l1i = l1i_start = per_cpu(current_l1i, cpu);
+ l2i_start = per_cpu(current_l2i, cpu);
+
+ if(l1 != 0)
+ {
+ masked_l1 = l1 & ((~0UL) << l1i);
+ if (masked_l1 != 0) {
+ l1i = __ffs(masked_l1);
+ if(l1i == l1i_start && l2i_start != 0) {
+ l2 = active_evtchns(cpu, s, l1i);
+ l2i = l2i_start;
+ l2_start = l2 & ((~0UL) >>
(BITS_PER_LONG - l2i));
+ }
+ }
+ else {
+ l1i = 0;
+ masked_l1 = l1 & ((~0UL) << l1i);
+ l1i = __ffs(masked_l1);
+ l2 = active_evtchns(cpu, s, l1i);
+ }
- while (l1 != 0) {
- masked_l1 = l1 & ((~0UL) << l1i);
- /* If we masked out all events, wrap to beginning. */
- if (masked_l1 == 0) {
- l1i = l2i = 0;
- continue;
- }
- l1i = __ffs(masked_l1);
+ while (1) {
- do {
- l2 = active_evtchns(cpu, s, l1i);
- masked_l2 = l2 & ((~0UL) << l2i);
- if (masked_l2 == 0)
- break;
- l2i = __ffs(masked_l2);
+ do {
+ masked_l2 = l2 & ((~0UL) << l2i);
+ if (masked_l2 == 0)
+ break;
+ l2i = __ffs(masked_l2);
- /* process port */
- port = (l1i * BITS_PER_LONG) + l2i;
- if ((irq = evtchn_to_irq[port]) != -1)
- do_IRQ(irq, regs);
- else
- evtchn_device_upcall(port);
+ /* process port */
+ port = (l1i * BITS_PER_LONG) + l2i;
+ if ((irq = evtchn_to_irq[port]) != -1)
+ do_IRQ(irq, regs);
+ else
+ evtchn_device_upcall(port);
- l2i = (l2i + 1) % BITS_PER_LONG;
+ l2i = (l2i + 1) % BITS_PER_LONG;
- /* Next caller starts at last processed + 1 */
- per_cpu(current_l1i, cpu) =
- l2i ? l1i : (l1i + 1) % BITS_PER_LONG;
- per_cpu(current_l2i, cpu) = l2i;
+ /* Next caller starts at last processed
+ 1 */
+ per_cpu(current_l1i, cpu) =
+ l2i ? l1i : (l1i + 1) %
BITS_PER_LONG;
+ per_cpu(current_l2i, cpu) = l2i;
- } while (l2i != 0);
+ } while (l2i != 0);
- l2 = active_evtchns(cpu, s, l1i);
- /* If we handled all ports, clear the selector bit. */
- if (l2 == 0)
l1 &= ~(1UL << l1i);
- l1i = (l1i + 1) % BITS_PER_LONG;
- l2i = 0;
+ if(l1 == 0)
+ break;
+
+ l1i = (l1i + 1) % BITS_PER_LONG;
+
+ masked_l1 = l1 & ((~0UL) << l1i);
+ /* If we masked out all events, wrap to
beginning. */
+ if (masked_l1 == 0) {
+ l1i = 0;
+ masked_l1 = l1 & ((~0UL) << l1i);
+ }
+ l1i = __ffs(masked_l1);
+ l2i = 0;
+ l2 = active_evtchns(cpu, s, l1i);
+ }
+
+ /* Check and process any pending events in the
+ * unprocessed portion of bits selected by l1i_start.
+ */
+ if(l2_start != 0) {
+ l1i = l1i_start;
+ l2i = 0;
+ do {
+ masked_l2 = l2_start & ((~0UL) << l2i);
+ if (masked_l2 == 0)
+ break;
+ l2i = __ffs(masked_l2);
+
+ /* process port */
+ port = (l1i * BITS_PER_LONG) + l2i;
+ if ((irq = evtchn_to_irq[port]) != -1)
+ do_IRQ(irq, regs);
+ else
+ evtchn_device_upcall(port);
+
+ l2i = (l2i + 1) % BITS_PER_LONG;
+
+ /* Next caller starts at last processed
+ 1 */
+ per_cpu(current_l1i, cpu) =
+ l2i ? l1i : (l1i + 1) %
BITS_PER_LONG;
+ per_cpu(current_l2i, cpu) = l2i;
+ }while(1);
+ }
}
/* If there were nested callbacks then we have more to do. */
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