| Date | Tue, 7 Nov 2023 20:08:03 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [RFC PATCH 00/86] Make the kernel preemptible |
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The kernel is not preemptible???? What are you smoking?
On Tue, 7 Nov 2023, Ankur Arora wrote:
> In voluntary models, the scheduler's job is to match the demand > side of preemption points (a task that needs to be scheduled) with > the supply side (a task which calls cond_resched().)
Voluntary preemption models are important for code optimization because the code can rely on the scheduler not changing the cpu we are running on. This allows removing code for preempt_enable/disable to be removed from the code and allows better code generation. The best performing code is generated with defined preemption points when we have a guarantee that the code is not being rescheduled on a different processor. This is f.e. important for consistent access to PER CPU areas.
> To do this add a new flag, TIF_NEED_RESCHED_LAZY which allows the > scheduler to mark that a reschedule is needed, but is deferred until > the task finishes executing in the kernel -- voluntary preemption > as it were.
That is different from the current no preemption model? Seems to be the same.
> There's just one remaining issue: now that explicit preemption points are > gone, processes that spread a long time in the kernel have no way to give > up the CPU.
These are needed to avoid adding preempt_enable/disable to a lot of primitives that are used for synchronization. You cannot remove those without changing a lot of synchronization primitives to always have to consider being preempted while operating.
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