Subject: Re: [PATCH RFC] Interface to set SPRN_TIDR

From: Sukadev Bhattiprolu <[email-masked]>
Date: Thu, 31 Aug 2017 11:06:18 -0700

felix [[email-masked]] wrote:
> On 31/08/2017 01:32, Sukadev Bhattiprolu wrote:
> > Michael Neuling [[email-masked]] wrote:
> > > Suka,
> > > 
> > > Please CC Christophe who as an alternative way of doing this. We ned to get
> > > agreement across all users of TIDR/AS_notify...
> > Mikey,
> > 
> > Thanks. There is overlap between the two patches. I will send a patch on
> > top of Christophe's for the interfaces to assign/clear the TIDR value and
> > clear the thread->tidr during arch_dup_task_struct(). I will also drop the
> > CONFIG_VAS check since its not only for VAS.
> > 
> > Christophe, can you let me know of any other comments on this patch?
> > 
> > Suka
> Suka,
> 
> I am seconding Christophe on this matter. I think that your patch now
> fulfills the CAPI use case requirements, with one exception: CAPI does not
> restrict assigning a thread id to the current task. Please find a few minor
> questions below.
> 
> Philippe
> 
> > > His patch is here:
> > >    https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.ozlabs.org_pipermail_linuxppc-2Ddev_2017-2DAugust_161582.html&d=DwIFAw&c=jf_iaSHvJObTbx-siA1ZOg&r=KC0fX9VGJYXlSiH9qN2ZONDbh8vUCZFX8GUhF3rHAvg&m=XQenBfWewOBjWopgf1Fh2UAVGnlzq766MNuzx7jYfuA&s=07WOVTh9f_4IBZfCJes4lvc7LWenBlqVfAXIXxL2QH4&e=
> > > 
> > > Mikey
> > > 
> > > On Tue, 2017-08-29 at 19:38 -0700, Sukadev Bhattiprolu wrote:
> > > > We need the SPRN_TIDR to be set for use with fast thread-wakeup
> > > > (core-to-core wakeup) in VAS. Each user thread that has a receive
> > > > window setup and expects to be notified when a sender issues a paste
> > > > needs to have a unique SPRN_TIDR value.
> > > > 
> > > > The SPRN_TIDR value only needs to unique within the process but for
> > > > now we use a globally unique thread id as described below.
> > > > 
> > > > Signed-off-by: Sukadev Bhattiprolu <[email-masked]>
> > > > ---
> > > > Changelog[v2]
> > > > 	- Michael Ellerman: Use an interface to assign TIDR so it is
> > > > 	  assigned to only threads that need it; move assignment to
> > > > 	  restore_sprs(). Drop lint from rebase;
> > > > 
> > > > 
> > > >   arch/powerpc/include/asm/processor.h |  4 ++
> > > >   arch/powerpc/include/asm/switch_to.h |  3 ++
> > > >   arch/powerpc/kernel/process.c        | 97
> > > > ++++++++++++++++++++++++++++++++++++
> > > >   3 files changed, 104 insertions(+)
> > > > 
> > > > diff --git a/arch/powerpc/include/asm/processor.h
> > > > b/arch/powerpc/include/asm/processor.h
> > > > index fab7ff8..bf6ba63 100644
> > > > --- a/arch/powerpc/include/asm/processor.h
> > > > +++ b/arch/powerpc/include/asm/processor.h
> > > > @@ -232,6 +232,10 @@ struct debug_reg {
> > > >   struct thread_struct {
> > > >   	unsigned long	ksp;		/* Kernel stack pointer */
> > > > 
> > > > +#ifdef CONFIG_PPC_VAS
> > > > +	unsigned long	tidr;
> > > > +#endif
> > > > +
> > > >   #ifdef CONFIG_PPC64
> > > >   	unsigned long	ksp_vsid;
> > > >   #endif
> > > > diff --git a/arch/powerpc/include/asm/switch_to.h
> > > > b/arch/powerpc/include/asm/switch_to.h
> > > > index 17c8380..4962455 100644
> > > > --- a/arch/powerpc/include/asm/switch_to.h
> > > > +++ b/arch/powerpc/include/asm/switch_to.h
> > > > @@ -91,4 +91,7 @@ static inline void clear_task_ebb(struct task_struct *t)
> > > >   #endif
> > > >   }
> > > > 
> > > > +extern void set_thread_tidr(struct task_struct *t);
> > > > +extern void clear_thread_tidr(struct task_struct *t);
> > > > +
> > > >   #endif /* _ASM_POWERPC_SWITCH_TO_H */
> > > > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > > > index 1f0fd36..13abb22 100644
> > > > --- a/arch/powerpc/kernel/process.c
> > > > +++ b/arch/powerpc/kernel/process.c
> > > > @@ -1132,6 +1132,10 @@ static inline void restore_sprs(struct thread_struct
> > > > *old_thread,
> > > >   			mtspr(SPRN_TAR, new_thread->tar);
> > > >   	}
> > > >   #endif
> > > > +#ifdef CONFIG_PPC_VAS
> > > > +	if (old_thread->tidr != new_thread->tidr)
> > > > +		mtspr(SPRN_TIDR, new_thread->tidr);
> > > > +#endif
> > > >   }
> > > > 
> > > >   #ifdef CONFIG_PPC_BOOK3S_64
> > > > @@ -1446,9 +1450,97 @@ void flush_thread(void)
> > > >   #endif /* CONFIG_HAVE_HW_BREAKPOINT */
> > > >   }
> > > > 
> > > > +#ifdef CONFIG_PPC_VAS
> > > > +static DEFINE_SPINLOCK(vas_thread_id_lock);
> > > > +static DEFINE_IDA(vas_thread_ida);
> > > > +
> > > > +/*
> > > > + * We need to assign an unique thread id to each thread in a process. This
> > > > + * thread id is intended to be used with the Fast Thread-wakeup (aka Core-
> > > > + * to-core wakeup) mechanism being implemented on top of Virtual Accelerator
> > > > + * Switchboard (VAS).
> > > > + *
> > > > + * To get a unique thread-id per process we could simply use task_pid_nr()
> > > > + * but the problem is that task_pid_nr() is not yet available for the thread
> > > > + * when copy_thread() is called. Fixing that would require changing more
> > > > + * intrusive arch-neutral code in code path in copy_process()?.
> > > > + *
> > > > + * Further, to assign unique thread ids within each process, we need an
> > > > + * atomic field (or an IDR) in task_struct, which again intrudes into the
> > > > + * arch-neutral code.
> > > > + *
> > > > + * So try to assign globally unique thraed ids for now.
> > > > + *
> > > > + * NOTE: TIDR 0 indicates that the thread does not need a TIDR value.
> > > > + * 	 For now, only threads that expect to be notified by the VAS
> > > > + * 	 hardware need a TIDR value and we assign values > 0 for those.
> > > > + */
> > > > +#define MAX_THREAD_CONTEXT	((1 << 15) - 2)
> Why are you excluding ((1 << 15) - 1)?

You are right. I don't need to exclude that. Also, TIDR is a 16-bit (0:15 in
VAS's Local Notify TID) value right? So, will change to

	#define MAX_THREAD_CONTEXT	((1 << 16) - 1)

> > > > +static int assign_thread_tidr(void)
> > > > +{
> > > > +	int index;
> > > > +	int err;
> > > > +
> > > > +again:
> > > > +	if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL))
> > > > +		return -ENOMEM;
> > > > +
> > > > +	spin_lock(&vas_thread_id_lock);
> > > > +	err = ida_get_new_above(&vas_thread_ida, 1, &index);
> Why are you excluding 1?

I think the "_above" in the name is misleading. The function header for
ida_get_new_above() says "above or equal" so 1 is not excluded?

> > > > +	spin_unlock(&vas_thread_id_lock);
> > > > +
> > > > +	if (err == -EAGAIN)
> > > > +		goto again;
> > > > +	else if (err)
> > > > +		return err;
> > > > +
> > > > +	if (index > MAX_THREAD_CONTEXT) {
> > > > +		spin_lock(&vas_thread_id_lock);
> > > > +		ida_remove(&vas_thread_ida, index);
> > > > +		spin_unlock(&vas_thread_id_lock);
> > > > +		return -ENOMEM;
> > > > +	}
> > > > +
> > > > +	return index;
> > > > +}
> > > > +
> > > > +static void free_thread_tidr(int id)
> > > > +{
> > > > +	spin_lock(&vas_thread_id_lock);
> > > > +	ida_remove(&vas_thread_ida, id);
> > > > +	spin_unlock(&vas_thread_id_lock);
> > > > +}
> > > > +
> > > > +void clear_thread_tidr(struct task_struct *t)
> > > > +{
> > > > +	if (t->thread.tidr) {
> > > > +		free_thread_tidr(t->thread.tidr);
> > > > +		t->thread.tidr = 0;
> > > > +		mtspr(SPRN_TIDR, 0);
> > > > +	}
> > > > +}
> > > > +
> > > > +/*
> > > > + * Assign an unique thread id for this thread and set it in the
> > > > + * thread structure. For now, we need this interface only for
> > > > + * the current task.
> > > > + */
> > > > +void set_thread_tidr(struct task_struct *t)
> > > > +{
> > > > +	WARN_ON(t != current);
> CAPI does not have this restriction. It should be possible to assign a
> thread id to an arbitrary task.

I see. We (or caller) just have to figure out the locking for the task.

> > > > +	t->thread.tidr = assign_thread_tidr();
> > > > +	mtspr(SPRN_TIDR, t->thread.tidr);
> > > > +}
> > > > +