Hi Jason,
Thanks for your reply.
I've succeeded in creating a packet sequence that replicates the problem
or something similar.
Basically the MemCtrl is stuck in a loop ignoring WRITEs and waiting for
READs that will never arrive.
In an hour or two, I'll isolate the sim object illustrating the problem
and post it as a github issue.
Regards,
- Olumide
On 29/07/2025 16:20, Jason Lowe-Power wrote:
Hi Olumide,
This sounds like a potential bug, though I'm a bit surprised this hasn't
been found before. I assume this is happening when you're trying to take
a checkpoint since it's occurring during the drain state.
Could you make an issue on the gem5 github issue page? https://
github.com/gem5/gem5/issues <https://github.com/gem5/gem5/issues>
Thanks!
Jason
On Mon, Jul 28, 2025 at 4:50 PM Olumide via gem5-dev <gem5-dev@gem5.org
<mailto:gem5-dev@gem5.org>> wrote:
Hi-
I'm repeatedly encountering situations where the MemCtrl stops
scheduling requests even though mem_intr->writeQueueSize is not zero.
Unfortunately I can neither reproduce it in a MWE or by capturing and
playing back a packet sequence. (As a last resort I may resort to time
travel debug capture with the rr utility.)
The problem:
In my simulations the path of execution often ends up on line 964 of
mem_ctrl.cc
https://github.com/gem5/gem5/blob/stable/src/mem/mem_ctrl.cc#L964
<https://github.com/gem5/gem5/blob/stable/src/mem/mem_ctrl.cc#L964>
where no more events are scheduled even though
(gdb) p mem_intr->writeQueueSize
$1 = 15
And
(gdb) call drainState()
$2 = gem5::DrainState::Running
(gdb) p writeLowThreshold
$3 = 32
(gdb) p mem_intr->writeQueueSize > writeLowThreshold
$4 = false
(gdb) p mem_intr->readQueueSize
$5 = 0
(gdb) p mem_intr->busState
$6 = gem5::memory::qos::MemCtrl::READ
I suspect the culprit is line 1127 of mem_ctrl.cc
https://github.com/gem5/gem5/blob/stable/src/mem/mem_ctrl.cc#L1127
<https://github.com/gem5/gem5/blob/stable/src/mem/mem_ctrl.cc#L1127>
The test:
(below_threshold && drainState() != DrainState::Draining)
returns true and switches the next bus state to READ even through
mem_intr->readQueueSize == 0.
When I explicitly force
mem_intr->busStateNext = MemCtrl::WRITE
MemCtrl processes one write message but sets the next bus state to
MemCtrl::READ. As such I've had to repeatedly force the next state to
MemCtrl::WRITE until mem_intr->writeQueueSize == 0.
Please advise.
Regards,
- Olumide
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