Re: Re: [PATCH v3] Fix one error in mthca_alloc

[康宁 <kangning18z@xxxxxxxxx>]

>
> On Fri, Aug 27, 2021 at 08:52:28AM +0800, kangning wrote:
> > drivers/infiniband/hw/mthca/mthca_allocator.c: alloc->last left unchanged in mthca_alloc, which
> > has impact on performance of function find_next_zero_bit in mthca_alloc.
> 
> I don't know what the sentence above means, but the change is unlikely
> to be correct.
> 
> When alloc->last starts to be equal to alloc->max, the
> find_next_zero_bit() will always return alloc->max. which will ensure
> that the following code is executed.
> 
>    48         if (obj >= alloc->max) {
>    49                 alloc->top = (alloc->top + alloc->max) & alloc->mask;
>    50                 obj = find_first_zero_bit(alloc->table, alloc->max);
>    51         }
> 

Thanks for your review.
Your analysis is right. However, this is a bitmap allocator for resource id allocation (like QP, and CQ). 
We first look at the situation where no resource id is released.
When the value of alloc->last starts to reaches alloc->max, the bitmap will 
be full. In this case, it's normal to return an invalid value.

Now, let's add resource releasing into consideration. 
The following code is part of mthca_free()：

70	spin_lock_irqsave(&alloc->lock, flags);
71
72	clear_bit(obj, alloc->table);
73	alloc->last = min(alloc->last, obj);
74	alloc->top = (alloc->top + alloc->max) & alloc->mask;

mthca_free() is used to release the allocated resource id. It will update alloc->last when obj is freed. 
So, if the bitmap has space for allocation, my modification can continuously work.

> 
> However the mthca_alloc() function has other error, it returns -1 while
> based on its declaration it needs to be unsigned,

I think you are right about this. But obj is the return value of u32 type, which is the requirement of resource id 
(though it may not fully use 32 bits). I have no idea of how to fix it.

Thanks.

> 
> Thanks
> 
> > 
&gt; &gt; Signed-off-by: kangning <kangning18z@xxxxxxxxx>
&gt; &gt; ---
&gt; &gt;  
&gt; &gt;  I squashed two commits into one in this version.
&gt; &gt;  
&gt; &gt;  drivers/infiniband/hw/mthca/mthca_allocator.c | 3 +++
&gt; &gt;  1 file changed, 3 insertions(+)
&gt; &gt; 
&gt; &gt; diff --git a/drivers/infiniband/hw/mthca/mthca_allocator.c b/drivers/infiniband/hw/mthca/mthca_allocator.c
&gt; &gt; index aef1d274a14e..1141695093e7 100644
&gt; &gt; --- a/drivers/infiniband/hw/mthca/mthca_allocator.c
&gt; &gt; +++ b/drivers/infiniband/hw/mthca/mthca_allocator.c
&gt; &gt; @@ -51,6 +51,9 @@ u32 mthca_alloc(struct mthca_alloc *alloc)
&gt; &gt;  	}
&gt; &gt;  
&gt; &gt;  	if (obj &lt; alloc-&gt;max) {
&gt; &gt; +		alloc-&gt;last = obj + 1;
&gt; &gt; +		if (alloc-&gt;last == alloc-&gt;max)
&gt; &gt; +			alloc-&gt;last = 0;
&gt; &gt;  		set_bit(obj, alloc-&gt;table);
&gt; &gt;  		obj |= alloc-&gt;top;
&gt; &gt;  	} else
&gt; &gt; -- 
&gt; &gt; 2.17.1
&gt; &gt; 
</kangning18z@xxxxxxxxx>