Re: [PATCH] coroutine: cap per-thread local pool size

[Daniel P . Berrangé]

On Tue, Mar 19, 2024 at 06:41:28PM +0100, Kevin Wolf wrote:
> Am 19.03.2024 um 18:10 hat Daniel P. Berrangé geschrieben:
> > On Tue, Mar 19, 2024 at 05:54:38PM +0100, Kevin Wolf wrote:
> > > Am 19.03.2024 um 14:43 hat Daniel P. Berrangé geschrieben:
> > > > On Mon, Mar 18, 2024 at 02:34:29PM -0400, Stefan Hajnoczi wrote:
> > > > > The coroutine pool implementation can hit the Linux vm.max_map_count
> > > > > limit, causing QEMU to abort with "failed to allocate memory for 
> > > > > stack"
> > > > > or "failed to set up stack guard page" during coroutine creation.
> > > > > 
> > > > > This happens because per-thread pools can grow to tens of thousands of
> > > > > coroutines. Each coroutine causes 2 virtual memory areas to be 
> > > > > created.
> > > > 
> > > > This sounds quite alarming. What usage scenario is justified in
> > > > creating so many coroutines?
> > > 
> > > Basically we try to allow pooling coroutines for as many requests as
> > > there can be in flight at the same time. That is, adding a virtio-blk
> > > device increases the maximum pool size by num_queues * queue_size. If
> > > you have a guest with many CPUs, the default num_queues is relatively
> > > large (the bug referenced by Stefan had 64), and queue_size is 256 by
> > > default. That's 16k potential requests in flight per disk.
> > 
> > If we have more than 1 virtio-blk device, does that scale up the max
> > coroutines too ?
> > 
> > eg would 32 virtio-blks devices imply 16k * 32 -> 512k potential
> > requests/coroutines ?
> 
> Yes. This is the number of request descriptors that fit in the
> virtqueues, and if you add another device with additional virtqueues,
> then obviously that increases the number of theoretically possible
> parallel requests.
> 
> The limits of what you can actually achieve in practice might be lower
> because I/O might complete faster than the time we need to process all
> of the queued requests, depending on how many vcpus are trying to
> "compete" with how many iothreads. Of course, the practical limits in
> five years might be different from today.
> 
> > > > IIUC, coroutine stack size is 1 MB, and so tens of thousands of
> > > > coroutines implies 10's of GB of memory just on stacks alone.
> > > 
> > > That's only virtual memory, though. Not sure how much of it is actually
> > > used in practice.
> > 
> > True, by default Linux wouldn't care too much about virtual memory,
> > Only if 'vm.overcommit_memory' is changed from its default, such
> > that Linux applies an overcommit ratio on RAM, then total virtual
> > memory would be relevant.
> 
> That's a good point and one that I don't have a good answer for, short
> of just replacing the whole QEMU block layer with rsd and switching to
> stackless coroutines/futures this way.
> 
> > > > > Eventually vm.max_map_count is reached and memory-related syscalls 
> > > > > fail.
> > > > 
> > > > On my system max_map_count is 1048576, quite alot higher than
> > > > 10's of 1000's. Hitting that would imply ~500,000 coroutines and
> > > > ~500 GB of stacks !
> > > 
> > > Did you change the configuration some time in the past, or is this just
> > > a newer default? I get 65530, and that's the same default number I've
> > > seen in the bug reports.
> > 
> > It turns out it is a Fedora change, rather than a kernel change:
> > 
> >   https://fedoraproject.org/wiki/Changes/IncreaseVmMaxMapCount
> 
> Good to know, thanks.
> 
> > > > > diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
> > > > > index 5fd2dbaf8b..2790959eaf 100644
> > > > > --- a/util/qemu-coroutine.c
> > > > > +++ b/util/qemu-coroutine.c
> > > > 
> > > > > +static unsigned int get_global_pool_hard_max_size(void)
> > > > > +{
> > > > > +#ifdef __linux__
> > > > > +    g_autofree char *contents = NULL;
> > > > > +    int max_map_count;
> > > > > +
> > > > > +    /*
> > > > > +     * Linux processes can have up to max_map_count virtual memory 
> > > > > areas
> > > > > +     * (VMAs). mmap(2), mprotect(2), etc fail with ENOMEM beyond 
> > > > > this limit. We
> > > > > +     * must limit the coroutine pool to a safe size to avoid running 
> > > > > out of
> > > > > +     * VMAs.
> > > > > +     */
> > > > > +    if (g_file_get_contents("/proc/sys/vm/max_map_count", &contents, 
> > > > > NULL,
> > > > > +                            NULL) &&
> > > > > +        qemu_strtoi(contents, NULL, 10, &max_map_count) == 0) {
> > > > > +        /*
> > > > > +         * This is a conservative upper bound that avoids exceeding
> > > > > +         * max_map_count. Leave half for non-coroutine users like 
> > > > > library
> > > > > +         * dependencies, vhost-user, etc. Each coroutine takes up 2 
> > > > > VMAs so
> > > > > +         * halve the amount again.
> > > > > +         */
> > > > > +        return max_map_count / 4;
> > > > 
> > > > That's 256,000 coroutines, which still sounds incredibly large
> > > > to me.
> > > 
> > > The whole purpose of the limitation is that you won't ever get -ENOMEM
> > > back, which will likely crash your VM. Even if this hard limit is high,
> > > that doesn't mean that it's fully used. Your setting of 1048576 probably
> > > means that you would never have hit the crash anyway.
> > > 
> > > Even the benchmarks that used to hit the problem don't even get close to
> > > this hard limit any more because the actual number of coroutines stays
> > > much smaller after applying this patch.
> > 
> > I'm more thinking about what's the worst case behaviour that a
> > malicious guest can inflict on QEMU, and cause unexpectedly high
> > memory usage in the host.
> > 
> > ENOMEM is bad for a friendy VM, but there's also the risk to the host
> > from a unfriendly VM exploiting the high limits
> 
> But from a QEMU perspective, what is the difference between a friendly
> high-performance VM that exhausts the available bandwidth to do its job
> as good and fast as possible, and a malicious VM that does that same
> just to waste host resources? I don't think QEMU can decide this, they
> look the same.
> 
> If you want a VM not to send 16k requests in parallel, you can configure
> its disk to expose less queues or a smaller queue size. The values I
> mentioned above are only defaults that allow friendly VMs to perform
> well out of the box, nothing prevents you from changing them to restrict
> the amount of resources a VM can use.

Reducing queues is a no-win scenario, as it limits the performance of
a single disk when used in isolation, in order to cap the worst case
when all disks are used concurrently :-( It would be nice to allow a
single disk to burst to a high level, and only limit coroutines if
many disks are all trying to concurrently burst to a high level.


With regards,
Daniel
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