Messages in this thread | | | Date | Wed, 28 Jul 2021 10:16:10 -0300 | From | Marcelo Tosatti <> | Subject | Re: [patch 1/4] add basic task isolation prctl interface |
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On Wed, Jul 28, 2021 at 01:55:33PM +0200, nsaenzju@redhat.com wrote: > Hi Marcelo, > > On Wed, 2021-07-28 at 06:37 -0300, Marcelo Tosatti wrote: > > On Wed, Jul 28, 2021 at 01:45:39AM +0200, Frederic Weisbecker wrote: > > > On Tue, Jul 27, 2021 at 11:52:09AM -0300, Marcelo Tosatti wrote: > > > > The meaning of isolated is specified as follows: > > > > > > > > Isolation features > > > > ================== > > > > > > > > - prctl(PR_ISOL_GET, ISOL_SUP_FEATURES, 0, 0, 0) returns the supported > > > > features as a return value. > > > > > > > > - prctl(PR_ISOL_SET, ISOL_FEATURES, bitmask, 0, 0) enables the features in > > > > the bitmask. > > > > > > > > - prctl(PR_ISOL_GET, ISOL_FEATURES, 0, 0, 0) returns the currently > > > > enabled features. > > > > > > So what are the ISOL_FEATURES here? A mode that we enter such as flush > > > vmstat _everytime_ we resume to userpace after (and including) this prctl() ? > > > > ISOL_FEATURES is just the "command" type (which you can get and set). > > > > The bitmask would include ISOL_F_QUIESCE_ON_URET, so: > > > > - bitmask = ISOL_F_QUIESCE_ON_URET; > > - prctl(PR_ISOL_SET, ISOL_FEATURES, bitmask, 0, 0) enables the features in > > the bitmask. > > > > - quiesce_bitmap = prctl(PR_ISOL_GET, PR_ISOL_SUP_QUIESCE_CFG, 0, 0, 0) > > (1) > > > > (returns the supported actions to be quiesced). > > > > - prctl(PR_ISOL_SET, PR_ISOL_QUIESCE_CFG, quiesce_bitmask, 0, 0) _sets_ > > the actions to be quiesced (2) > > > > If an application does not modify "quiesce_bitmask" between > > points (1) and (2) above, it will enable quiescing of all > > "features" the kernel supports. > > I think this pattern of enabling all by default might be prone to subtly > breaking things.
The reasoning behind this pattern is that many latency sensitive applications (as far as i am aware) prefer "as few interruptions as possible, no interruptions is preferred".
In that case, the pattern makes sense.
> For example, let's say we introduce ISOL_F_QUIESCE_DEFER_TLB_FLUSH, this will > defer relatively short IPIs on isolated CPUs in exchange for a longer flush > whenever we enter the kernel (syscall, IRQs, NMI, etc...).
Why the flush has to be longer when you enter the kernel?
ISOL_F_QUIESCE_DEFER_TLB_FLUSH might collapse multiple IPIs into a single IPI, so the behaviour might be beneficial for "standard" types of application as well.
> A latency sensitive > application might be OK with the former but not with the latter.
Two alternatives:
1) The pattern above, where particular subsystems that might interrupt the kernel are enabled automatically if the kernel supports it.
Pros: Applications which implement this only need to be changed once, and can benefit from new kernel features.
Applications can disable particular features if they turn out to be problematic.
Cons: New features might break applications.
2) Force applications to enable each new feature individually.
Pros: Won't cause regressions, kernel behaviour is explicitly controlled by userspace.
Cons: Apps won't benefit from new features automatically.
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It seems to me 1) is preferred. Can also add a sysfs control to have a "default_isolation_feature" flag, which can be changed by a sysadmin in case a new feature is undesired.
Thoughts?
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