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SubjectRe: [RFC PATCH 14/16] irq: Add support for core-wide protection of IRQ and softirq
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Hi Joel/Vineeth,

On 2020/7/1 5:32, Vineeth Remanan Pillai wrote:
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
>
> With current core scheduling patchset, non-threaded IRQ and softirq
> victims can leak data from its hyperthread to a sibling hyperthread
> running an attacker.
>
> For MDS, it is possible for the IRQ and softirq handlers to leak data to
> either host or guest attackers. For L1TF, it is possible to leak to
> guest attackers. There is no possible mitigation involving flushing of
> buffers to avoid this since the execution of attacker and victims happen
> concurrently on 2 or more HTs.
>
> The solution in this patch is to monitor the outer-most core-wide
> irq_enter() and irq_exit() executed by any sibling. In between these
> two, we mark the core to be in a special core-wide IRQ state.
>
> In the IRQ entry, if we detect that the sibling is running untrusted
> code, we send a reschedule IPI so that the sibling transitions through
> the sibling's irq_exit() to do any waiting there, till the IRQ being
> protected finishes.
>
> We also monitor the per-CPU outer-most irq_exit(). If during the per-cpu
> outer-most irq_exit(), the core is still in the special core-wide IRQ
> state, we perform a busy-wait till the core exits this state. This
> combination of per-cpu and core-wide IRQ states helps to handle any
> combination of irq_entry()s and irq_exit()s happening on all of the
> siblings of the core in any order.
>
> Lastly, we also check in the schedule loop if we are about to schedule
> an untrusted process while the core is in such a state. This is possible
> if a trusted thread enters the scheduler by way of yielding CPU. This
> would involve no transitions through the irq_exit() point to do any
> waiting, so we have to explicitly do the waiting there.
>
> Every attempt is made to prevent a busy-wait unnecessarily, and in
> testing on real-world ChromeOS usecases, it has not shown a performance
> drop. In ChromeOS, with this and the rest of the core scheduling
> patchset, we see around a 300% improvement in key press latencies into
> Google docs when Camera streaming is running simulatenously (90th
> percentile latency of ~150ms drops to ~50ms).
>
> This fetaure is controlled by the build time config option
> CONFIG_SCHED_CORE_IRQ_PAUSE and is enabled by default. There is also a
> kernel boot parameter 'sched_core_irq_pause' to enable/disable the
> feature at boot time. Default is enabled at boot time.

We saw a lot of soft lockups on the screen when we tested v6.

[ 186.527883] watchdog: BUG: soft lockup - CPU#86 stuck for 22s! [uperf:5551]
[ 186.535884] watchdog: BUG: soft lockup - CPU#87 stuck for 22s! [uperf:5444]
[ 186.555883] watchdog: BUG: soft lockup - CPU#89 stuck for 22s! [uperf:5547]
[ 187.547884] rcu: INFO: rcu_sched self-detected stall on CPU
[ 187.553760] rcu: 40-....: (14997 ticks this GP) idle=49a/1/0x4000000000000002 softirq=1711/1711 fqs=7279
[ 187.564685] NMI watchdog: Watchdog detected hard LOCKUP on cpu 14
[ 187.564723] NMI watchdog: Watchdog detected hard LOCKUP on cpu 38

The problem is gone when we reverted this patch. We are running multiple
uperf threads(equal to cpu number) in a cgroup with coresched enabled.
This is 100% reproducible on our side.

Just wonder if anything already known before we dig into it.

Thanks,
-Aubrey




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