Re: [PATCH v6 06/17] firmware: arm_scmi: Introduce monotonically increasing tokens

[Sudeep Holla]

On Wed, Jul 28, 2021 at 05:54:30PM +0100, Cristian Marussi wrote:
> On Wed, Jul 28, 2021 at 03:17:46PM +0100, Sudeep Holla wrote:
> > On Mon, Jul 12, 2021 at 03:18:22PM +0100, Cristian Marussi wrote:

[...]

> > >  
> > > +#define SCMI_PENDING_XFERS_HT_ORDER_SZ	9
> > > +
> > 
> > Is there any particular reason to choose half the token size as hash bucket
> > size ? IOW why not 1/3 or 1/4th of it ? I would appreciate a comment here.
> > I see it is mentioned in the commit log. Also is it not better to associate
> > or keep it close to MSG_TOKEN_ID_MASK and associated macros.
> > 
> 
> I'll move this in the proper place where associated macros are defined.
> 
> The reason for the size choice is tricky (and not sure about its value
> still...so I have not commented yet :D); the ideal size of this hashtable would
> be desc->max_msg so equal to the maximum number of inflight messages allowed on
> the system in order to minimize (probably to zero) collisions on the hashtable:
> unfortunately max_msg is only finally available at runtime time and the
> kernel hashtable is statically sized by design....
> 
> I tried to play some tricks to define dynamically the size but everything falls
> apart since a lot of stuff in linux/hashtable.h is based on ARRAY_SIZE() and
> friends (to speedup all I suppose). Another non-fit (in my opinion)
> alternative would be using relativistic hashtable (linux/rhashtable.h) but
> those are definitely overkill in our case since they are hashtables that
> can be resized completely at runtime while populated O_o. (with even
> more overhead)
> 
> At the end the size that fits all possible in-flight messages minimizing
> collisions in any possible case that I can set at compile time would be 10,
> which means really 2^10 1024 HT entries (equal to MAX_MSG_TOKEN) each of which
> is a struct list_head (*prev,*next 16bytes) i.e. 16KB HT: Peter pointed out
> that it would be a lot of wasted space on normal systems in which max in-flight
> messages are far-less than 1024 AND would not even fit in one 4Kb page, so I
> reduced it to 512 entries but the best would be 256 (8) if we want to
> fit in one regular 4kb page. The drawback will be a bit of HT collisions on
> system with more than 256 possible and effective in-flight messages.
>

I agree, 256 should be fine for now. Just add a note that it is chosen to
fit a page and can be updated if required.


> > >  /**
> > >   * struct scmi_xfers_info - Structure to manage transfer information
> > >   *
> > > - * @xfer_block: Preallocated Message array
> > >   * @xfer_alloc_table: Bitmap table for allocated messages.
> > >   *	Index of this bitmap table is also used for message
> > >   *	sequence identifier.
> > >   * @xfer_lock: Protection for message allocation
> > > + * @last_token: A counter to use as base to generate for monotonically
> > > + *		increasing tokens.
> > > + * @free_xfers: A free list for available to use xfers. It is initialized with
> > > + *		a number of xfers equal to the maximum allowed in-flight
> > > + *		messages.
> > > + * @pending_xfers: An hashtable, indexed by msg_hdr.seq, used to keep all the
> > > + *		   currently in-flight messages.
> > >   */
> > >  struct scmi_xfers_info {
> > > -	struct scmi_xfer *xfer_block;
> > >  	unsigned long *xfer_alloc_table;
> > >  	spinlock_t xfer_lock;
> > > +	atomic_t last_token;
> > 
> > Can we merge this and transfer_last_id ? Let this be free running like
> > transfer_last_id and just use [0-9] from this ? I don't see any point
> > having 2 different monotonically increasing tokens/id.
> > 
> 
> Mmm I was tempted about that, but the reason I did not was that in some
> rare limit condition as you can see in the ASCII art (:O) I can find a hole in
> the next available token ids so I have to skip and update last_token itself,
> not sure if this could cause confusion seeing transfer_ids with holes during
> tracing if I unify them.
>

That should be fine as it won't be used at all.

-- 
Regards,
Sudeep