On 10/31/2014 11:19 AM, Bart Van Assche wrote:
On 10/30/14 18:33, Sagi Grimberg wrote:
Now I realize that we can hit serious problems here since we never
solved the issue of srp polling routine that might poll forever within
an interrupt (or at least until a hard lockup). Its interesting that
you weren't able to hit that with a high workload. Did you try running
this code on a virtual function (I witnessed this issue in iser on a VM).
Moreover, the fairness issue is even more likely to be encountered in
multichannel. Did you try to hit that? I really think this patchset
*needs* to deal with the 2 issues I mentioned as the probability of
hitting them increases with a faster IO stack.
I remember this was discussed lately with consideration for using
blk-iopoll or not. But I think that for now the initial approach of
bailing out of the once we hit a budget is fine for now.
Hello Sagi,
As you mentioned so far this fairness issue has only caused trouble with
iSER in a virtual machine guest. I have not yet seen anyone reporting a
QP servicing fairness problem for the SRP initiator.
IMHO, this is not iSER specific issue, it is easily indicated from the
code that a specific workload SRP will poll recv completion queue
forever in an interrupt context.
I encountered this issue on a virtual guest in a high workload (80+
sessions with heavy traffic on all) because qemu smp_affinity setting
was broken (might still be, didn't check that for a while). This caused
all completion vectors to fire interrupts to core 0 causing a high
events contention on a single event queue (causing lockup situations
and starvation of other CQs). Using more completion queues will enhance
this situation.
I think running multichannel code when all MSIX vectors affinity are
directed to a single CPU can invoke what I'm talking about.
Although analyzing
and if needed limiting the maximum number of iterations in the SRP
polling routine is on my to-do list, addressing that issue is outside of
the scope of this patch series.
Although both of us did not yet hear of such complaints from SRP users,
I disagree because this might make the problems worse. But if you want
to take it later I guess that's fine too.
Regarding the impact of this patch series on QP handling fairness: the
time spent in the SRP RDMA completion handler depends on the number of
completions processed at once. This number depends on:
(a) The number of CPU cores in the initiator system that submit I/O and
that are associated with a single RDMA channel.
(b) The target system processing speed per RDMA channel.
This patch series reduces (a) by a factor ch_count.
This is under the assumption that IRQ affinity is spread across several
CPUS and that's fine, but we should *not* hit a hard lockup in case it
is not (and I suspect we can).
(b) is either
unaffected (linear scaling) or slightly reduced (less than linear
scaling). My conclusion is that if this patch series has an impact on QP
handling fairness that it will improve fairness since the number of
completions processed at once either remains unchanged or that it is
reduced.
I think in the single CPU completion queue processing, this can enhance
the problem as well.
Sagi.
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