Re: [PATCH v0.8 3/6] sched/umcg: implement UMCG syscalls

Tao Zhou
2021-11-08T06:56:57Z

On Sun, Nov 07, 2021 at 10:26:34AM -0800, Peter Oskolkov wrote:

> On Sat, Nov 6, 2021 at 10:19 AM Tao Zhou <tao.zhou@linux.dev> wrote:
> >
> > Hi Peter,
> >
> > On Thu, Nov 04, 2021 at 12:58:01PM -0700, Peter Oskolkov wrote:
> >
> > > +/**
> > > + * umcg_update_state: atomically update umcg_task.state_ts, set new timestamp.
> > > + * @state_ts - points to the state_ts member of struct umcg_task to update;
> > > + * @expected - the expected value of state_ts, including the timestamp;
> > > + * @desired - the desired value of state_ts, state part only;
> > > + * @may_fault - whether to use normal or _nofault cmpxchg.
> > > + *
> > > + * The function is basically cmpxchg(state_ts, expected, desired), with extra
> > > + * code to set the timestamp in @desired.
> > > + */
> > > +static int umcg_update_state(u64 __user *state_ts, u64 *expected, u64 desired,
> > > + bool may_fault)
> > > +{
> > > + u64 curr_ts = (*expected) >> (64 - UMCG_STATE_TIMESTAMP_BITS);
> > > + u64 next_ts = ktime_get_ns() >> UMCG_STATE_TIMESTAMP_GRANULARITY;
> > > +
> > > + /* Cut higher order bits. */
> > > + next_ts &= UMCG_TASK_STATE_MASK_FULL;
> >
> > next_ts &= (1 << UMCG_STATE_TIMESTAMP_BITS) - 1; or am I wrong.
>
> Right, thanks. I'll fix it in the next patchset version, if any. But
> at the moment I don't think this is bad enough to prevent merging, if
> the maintainers feel like it - basically, the condition below will
> always be false, so if the state is updated within 16 nanoseconds, the
> timestamps may sometimes match. For this to be an issue, this should
> result in ABA updates, so two state changes should happen in 16
> nanoseconds, which is extremely unlikely (impossible?), as most state

The task state occupy 0-5 bits and use 2 bits(00, 01, 10, 11) to denote
NONE, RUNNING, IDLE, BLOCK. Is it possible to grasp 4 bits from here to
used to extend the timestamp resolution.

> changes are accompanied by other atomic ops.
>
> >
> > > + if (next_ts == curr_ts)
> > > + ++next_ts;
> > > +
> > > + /* Remove an old timestamp, if any. */
> > > + desired &= UMCG_TASK_STATE_MASK_FULL;
> > > +
> > > + /* Set the new timestamp. */
> > > + desired |= (next_ts << (64 - UMCG_STATE_TIMESTAMP_BITS));
> > > +
> > > + if (may_fault)
> > > + return cmpxchg_user_64(state_ts, expected, desired);
> > > +
> > > + return cmpxchg_user_64_nofault(state_ts, expected, desired);
> > > +}
> > > +
> > > +/**
> > > + * sys_umcg_ctl: (un)register the current task as a UMCG task.
> > > + * @flags: ORed values from enum umcg_ctl_flag; see below;
> > > + * @self: a pointer to struct umcg_task that describes this
> > > + * task and governs the behavior of sys_umcg_wait if
> > > + * registering; must be NULL if unregistering.
> > > + *
> > > + * @flags & UMCG_CTL_REGISTER: register a UMCG task:
> > > + * UMCG workers:
> > > + * - @flags & UMCG_CTL_WORKER
> > > + * - self->state must be UMCG_TASK_BLOCKED
> > > + * UMCG servers:
> > > + * - !(@flags & UMCG_CTL_WORKER)
> > > + * - self->state must be UMCG_TASK_RUNNING
> > > + *
> > > + * All tasks:
> > > + * - self->next_tid must be zero
> > > + *
> > > + * If the conditions above are met, sys_umcg_ctl() immediately returns
> > > + * if the registered task is a server; a worker will be added to
> > > + * idle_workers_ptr, and the worker put to sleep; an idle server
> > > + * from idle_server_tid_ptr will be woken, if present.
> > > + *
> > > + * @flags == UMCG_CTL_UNREGISTER: unregister a UMCG task. If the current task
> > > + * is a UMCG worker, the userspace is responsible for waking its
> > > + * server (before or after calling sys_umcg_ctl).
> > > + *
> > > + * Return:
> > > + * 0 - success
> > > + * -EFAULT - failed to read @self
> > > + * -EINVAL - some other error occurred
> > > + */
> > > +SYSCALL_DEFINE2(umcg_ctl, u32, flags, struct umcg_task __user *, self)
> > > +{
> > > + struct umcg_task ut;
> > > +
> > > + if (flags == UMCG_CTL_UNREGISTER) {
> > > + if (self || !current->umcg_task)
> > > + return -EINVAL;
> > > +
> > > + if (current->flags & PF_UMCG_WORKER)
> > > + umcg_handle_exiting_worker();
> > > + else
> > > + umcg_clear_task(current);
> > > +
> > > + return 0;
> > > + }
> > > +
> > > + if (!(flags & UMCG_CTL_REGISTER))
> > > + return -EINVAL;
> > > +
> > > + flags &= ~UMCG_CTL_REGISTER;
> > > + if (flags && flags != UMCG_CTL_WORKER)
> > > + return -EINVAL;
> > > +
> > > + if (current->umcg_task || !self)
> > > + return -EINVAL;
> > > +
> > > + if (copy_from_user(&ut, self, sizeof(ut)))
> > > + return -EFAULT;
> > > +
> > > + if (ut.next_tid)
> > > + return -EINVAL;
> > > +
> > > + if (flags == UMCG_CTL_WORKER) {
> > > + if ((ut.state_ts & UMCG_TASK_STATE_MASK_FULL) != UMCG_TASK_BLOCKED)
> >
> > Or use UMCG_TASK_STATE_MASK that is enough.
>
> Do you have a use case for this (i.e. when state flags can be
> legitimately set here)? At the moment I can't think of it, and I'd
> rather keep things more strict to avoid dealing with unexpected use
> cases in the future.

When read through this thread, I am not realize that this time the
state flags should not be set. But I need to go other round to be more
clear like I'm now reading the doc again..

> > > + return -EINVAL;
> > > +
> > > + WRITE_ONCE(current->umcg_task, self);
> > > + current->flags |= PF_UMCG_WORKER;
> > > +
> > > + /* Trigger umcg_handle_resuming_worker() */
> > > + set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
> > > + } else {
> > > + if ((ut.state_ts & UMCG_TASK_STATE_MASK_FULL) != UMCG_TASK_RUNNING)
> >
> > The same here.
>
> Yes, the same here - why do you think task state flags should be allowed here?
>
>
> >
> > > + return -EINVAL;
> > > +
> > > + WRITE_ONCE(current->umcg_task, self);
> > > + }
> > > +
> > > + return 0;
> > > +}
> > > +
> >
> >
> > Thanks,
> > Tao