Re: [PATCH 01/16] rcu/tree: Add a work to allocate pages from regular context

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Nov 03, 2020 at 12:54:22PM -0500, Joel Fernandes wrote:
> On Thu, Oct 29, 2020 at 05:50:04PM +0100, Uladzislau Rezki (Sony) wrote:
> > The current memmory-allocation interface presents to following
> > difficulties that this patch is designed to overcome
> [...]
> > ---
> >  kernel/rcu/tree.c | 109 ++++++++++++++++++++++++++++------------------
> >  1 file changed, 66 insertions(+), 43 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 06895ef85d69..f2da2a1cc716 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -177,7 +177,7 @@ module_param(rcu_unlock_delay, int, 0444);
> >   * per-CPU. Object size is equal to one page. This value
> >   * can be changed at boot time.
> >   */
> > -static int rcu_min_cached_objs = 2;
> > +static int rcu_min_cached_objs = 5;
> >  module_param(rcu_min_cached_objs, int, 0444);
> >  
> >  /* Retrieve RCU kthreads priority for rcutorture */
> > @@ -3084,6 +3084,9 @@ struct kfree_rcu_cpu_work {
> >   *	In order to save some per-cpu space the list is singular.
> >   *	Even though it is lockless an access has to be protected by the
> >   *	per-cpu lock.
> > + * @page_cache_work: A work to refill the cache when it is empty
> > + * @work_in_progress: Indicates that page_cache_work is running
> > + * @hrtimer: A hrtimer for scheduling a page_cache_work
> >   * @nr_bkv_objs: number of allocated objects at @bkvcache.
> >   *
> >   * This is a per-CPU structure.  The reason that it is not included in
> > @@ -3100,6 +3103,11 @@ struct kfree_rcu_cpu {
> >  	bool monitor_todo;
> >  	bool initialized;
> >  	int count;
> > +
> > +	struct work_struct page_cache_work;
> > +	atomic_t work_in_progress;
> 
> Does it need to be atomic? run_page_cache_work() is only called under a lock.
> You can use xchg() there. And when you do the atomic_set, you can use
> WRITE_ONCE as it is a data-race.
> 
We can use xchg together with *_ONCE() macro. Could you please clarify what
is your concern about using atomic_t? Both xchg() and atomic_xchg() guarantee
atamarity. Same as WRITE_ONCE() or atomic_set().

> > @@ -4449,24 +4482,14 @@ static void __init kfree_rcu_batch_init(void)
> >  
> >  	for_each_possible_cpu(cpu) {
> >  		struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> > -		struct kvfree_rcu_bulk_data *bnode;
> >  
> >  		for (i = 0; i < KFREE_N_BATCHES; i++) {
> >  			INIT_RCU_WORK(&krcp->krw_arr[i].rcu_work, kfree_rcu_work);
> >  			krcp->krw_arr[i].krcp = krcp;
> >  		}
> >  
> > -		for (i = 0; i < rcu_min_cached_objs; i++) {
> > -			bnode = (struct kvfree_rcu_bulk_data *)
> > -				__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > -
> > -			if (bnode)
> > -				put_cached_bnode(krcp, bnode);
> > -			else
> > -				pr_err("Failed to preallocate for %d CPU!\n", cpu);
> > -		}
> > -
> >  		INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> > +		INIT_WORK(&krcp->page_cache_work, fill_page_cache_func);
> >  		krcp->initialized = true;
> 
> During initialization, is it not better to still pre-allocate? That way you
> don't have to wait to get into a situation where you need to initially
> allocate.
> 
Since we have a worker that does it when a cache is empty there is no
a high need in doing it during initialization phase. If we can reduce
an amount of code it is always good :)

Thanks, Joel.

--
Vlad Rezki