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qemu: Do not error out when setting affinity failed

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Consider a host with 8 CPUs. There are the following possible scenarios

1. Bare metal; libvirtd has affinity of 8 CPUs; QEMU should get 8 CPUs

2. Bare metal; libvirtd has affinity of 2 CPUs; QEMU should get 8 CPUs

3. Container has affinity of 8 CPUs; libvirtd has affinity of 8 CPus;
   QEMU should get 8 CPUs

4. Container has affinity of 8 CPUs; libvirtd has affinity of 2 CPus;
   QEMU should get 8 CPUs

5. Container has affinity of 4 CPUs; libvirtd has affinity of 4 CPus;
   QEMU should get 4 CPUs

6. Container has affinity of 4 CPUs; libvirtd has affinity of 2 CPus;
   QEMU should get 4 CPUs

Scenarios 1 & 2 always work unless systemd restricted libvirtd privs.

Scenario 3 works because libvirt checks current affinity first and
skips the sched_setaffinity call, avoiding the SYS_NICE issue

Scenario 4 works only if CAP_SYS_NICE is availalbe

Scenarios 5 & 6 works only if CAP_SYS_NICE is present *AND* the cgroups
cpuset is not set on the container.

If libvirt blindly ignores the sched_setaffinity failure, then scenarios
4, 5 and 6 should all work, but with caveat in case 4 and 6, that
QEMU will only get 2 CPUs instead of the possible 8 and 4 respectively.
This is still better than failing.

Therefore libvirt can blindly ignore the setaffinity failure, but *ONLY*
ignore it when there was no affinity specified in the XML config.
If user specified affinity explicitly, libvirt must report an error if
it can't be honoured.

Resolves: https://bugzilla.redhat.com/1819801

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
This commit is contained in:
Martin Kletzander 2020-09-04 14:17:30 +02:00
parent 59c5bf3faa
commit 3791f29b08
1 changed files with 37 additions and 3 deletions
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40
src/qemu/qemu_process.c
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@ -2528,6 +2528,7 @@ qemuProcessGetAllCpuAffinity(virBitmapPtr *cpumapRet)
static int
qemuProcessInitCpuAffinity(virDomainObjPtr vm)
{
bool settingAll = false;
g_autoptr(virBitmap) cpumapToSet = NULL;
virDomainNumatuneMemMode mem_mode;
qemuDomainObjPrivatePtr priv = vm->privateData;
@ -2566,13 +2567,29 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm)
if (!(cpumapToSet = virBitmapNewCopy(vm->def->cputune.emulatorpin)))
return -1;
} else {
settingAll = true;
if (qemuProcessGetAllCpuAffinity(&cpumapToSet) < 0)
return -1;
}
if (cpumapToSet &&
virProcessSetAffinity(vm->pid, cpumapToSet) < 0) {
return -1;
/*
* We only want to error out if we failed to set the affinity to
* user-requested mapping. If we are just trying to reset the affinity
* to all CPUs and this fails it can only be an issue if:
* 1) libvirtd does not have CAP_SYS_NICE
* 2) libvirtd does not run on all CPUs
*
* This scenario can easily occurr when libvirtd is run inside a
* container with restrictive permissions and CPU pinning.
*
* See also: https://bugzilla.redhat.com/1819801#c2
*/
if (settingAll)
virResetLastError();
else
return -1;
}
return 0;
@ -2726,8 +2743,25 @@ qemuProcessSetupPid(virDomainObjPtr vm,
affinity_cpumask = use_cpumask;
/* Setup legacy affinity. */
if (affinity_cpumask && virProcessSetAffinity(pid, affinity_cpumask) < 0)
goto cleanup;
if (affinity_cpumask && virProcessSetAffinity(pid, affinity_cpumask) < 0) {
/*
* We only want to error out if we failed to set the affinity to
* user-requested mapping. If we are just trying to reset the affinity
* to all CPUs and this fails it can only be an issue if:
* 1) libvirtd does not have CAP_SYS_NICE
* 2) libvirtd does not run on all CPUs
*
* However since this scenario is very improbable, we rather skip
* reporting the error because it helps running libvirtd in a a scenario
* where pinning is handled by someone else.
*
* See also: https://bugzilla.redhat.com/1819801#c2
*/
if (affinity_cpumask == hostcpumap)
virResetLastError();
else
goto cleanup;
}
/* Set scheduler type and priority, but not for the main thread. */
if (sched &&