Re: [libvirt] [PATCH 1/2] qemu: fix recording of vCPU pids for MTTCG

On Mon, Oct 29, 2018 at 05:55:36PM -0400, John Ferlan wrote: > > > On 10/17/18 10:15 AM, Daniel P. Berrangé wrote: >> MTTCG is the new multi-threaded impl of TCG which follows >> KVM in having one host OS thread per vCPU. Historically >> we have discarded all PIDs reported for TCG guests, but >> we must now selectively honour this data. >> >> We don't have anything in the domain XML that indicates >> whether a guest is using TCG or MTTCG. While QEMU does >> have an option (-accel tcg,thread=single|multi), it is >> not desirable to expose this in libvirt. QEMU will >> automatically use MTTCG when the host/guest architecture >> pairing is known to be safe. Only developers of QEMU TCG >> have a strong reason to override this logic. >> >> Thus we use two sanity checks to decide if the vCPU >> PID information is usable. First we see if the PID >> duplicates the main emulator PID, and second we see >> if the PID duplicates any other vCPUs. >> >> Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; >> --- >> src/qemu/qemu_domain.c | 81 ++++++++++++++++++++++++++---------------- >> 1 file changed, 51 insertions(+), 30 deletions(-) >> >> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c >> index f00f1b3fdb..c7a0c03e3f 100644 >> --- a/src/qemu/qemu_domain.c >> +++ b/src/qemu/qemu_domain.c >> @@ -10326,9 +10326,10 @@ qemuDomainRefreshVcpuInfo(virQEMUDriverPtr driver, >> qemuDomainVcpuPrivatePtr vcpupriv; >> qemuMonitorCPUInfoPtr info = NULL; >> size_t maxvcpus = virDomainDefGetVcpusMax(vm->def); >> - size_t i; >> + size_t i, j; >> bool hotplug; >> bool fast; >> + bool validTIDs = true; >> int rc; >> int ret = -1; >> >> @@ -10336,6 +10337,8 @@ qemuDomainRefreshVcpuInfo(virQEMUDriverPtr driver, >> fast = virQEMUCapsGet(QEMU_DOMAIN_PRIVATE(vm)->qemuCaps, >> QEMU_CAPS_QUERY_CPUS_FAST); >> >> + VIR_DEBUG("Maxvcpus %zu hotplug %d fast query %d", maxvcpus, hotplug, fast); >> + >> if (qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) < 0) >> return -1; >> >> @@ -10348,39 +10351,57 @@ qemuDomainRefreshVcpuInfo(virQEMUDriverPtr driver, >> if (rc < 0) >> goto cleanup; >> >> + /* >> + * The query-cpus[-fast] commands return information >> + * about the vCPUs, including the OS level PID that >> + * is executing the vCPU. >> + * >> + * For KVM there is always a 1-1 mapping between >> + * vCPUs and host OS PIDs. >> + * >> + * For TCG things are a little more complicated. >> + * >> + * - In some cases the vCPUs will all have the same >> + * PID as the main emulator thread. >> + * - In some cases the first vCPU will have a distinct >> + * PID, but other vCPUs will share the emulator thread >> + * >> + * For MTTCG, things work the same as KVM, with each >> + * vCPU getting its own PID. >> + * >> + * We use the Host OS PIDs for doing vCPU pinning >> + * and reporting. The TCG data reporting will result >> + * in bad behaviour such as pinning the wrong PID. >> + * We must thus detect and discard bogus PID info >> + * from TCG, while still honouring the modern MTTCG >> + * impl which we can support. >> + */ >> + for (i = 0; i < maxvcpus && validTIDs; i++) { >> + if (info[i].tid == vm->pid) { >> + VIR_DEBUG("vCPU[%zu] PID %llu duplicates process", >> + i, (unsigned long long)info[i].tid); >> + validTIDs = false; >> + } >> + > > If !validTIDs does the next loop matter? IOW: > > Should the above section add a "continue;" since the loop exit would > force the exit? Mostly I wanted to ensure that we logged all the vCPU pids and it won't impose an unreasonable performance impact by doing so.

> Beyond that the logic and comments look reasonable. I assume since > domain XML doesn't care whether MTTCG or TCG is used and things are > handled under the covers by QEMU that means there's no migration or > save/restore issues. Of course you have a much deeper understanding of > the QEMU code than I do! > > The one other question I'd have is should validTIDs setting be done just > once and saved perhaps in the domain private block? There is more than 1 > caller (and *Launch can call twice). It's not like it's going to change, > right? So doing the same loop from a hotplug path won't matter nor would > subsequent reconnects or attaches. So perhaps the validTIDs should be a > tristate that only needs to be checked when the value is UNKNOWN. It's > not like the loop is that expensive since it's only numeric comparisons, > so it doesn't matter. I suppose I can be easily convinced taking this > route would be fine, but figured I'd ask. AFAIK, this can only be called once per running VM for each libvirtd run, so I dont see need to do any more advanced caching.