# Re: Re: \[PATCH v3\] Fix one error in mthca\_alloc - From: \ - Date: 2021-08-31 12:17:13 - List: linux-rdma - Page: https://ratatoskr.run/linux-rdma/2021/08/15464842 - Thread: https://ratatoskr.run/linux-rdma/2021/08/15464821/t.md ```text > On Tue, Aug 31, 2021 at 06:40:38PM +0800, 康宁 wrote: > > > > > > 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. > > Yes, 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. > > After alloc->last starts to be equal to alloc->max, the bitmap is > searches with find_first_zero_bit() call. That will ensure that we look > for any bit between 0 and alloc->max. > > So no, your change can't be right. If you never call function mthca_free(), you will never find a valid bit in [0, alloc->max] with find_first_zero_bit() call. Cause every bits between 0 and alloc->max are alloacted. But if you called mthca_free() before, the freed obj is assigned to alloc->last, and we still can allocate the bitmap. My modification is correct. > > Thanks > > > > > > > > > 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 > > > > > > > > > > > Signed-off-by: kangning [ref] > > > > --- > > > > > > > > I squashed two commits into one in this version. > > > > > > > > drivers/infiniband/hw/mthca/mthca_allocator.c | 3 +++ > > > > 1 file changed, 3 insertions(+) > > > > > > > > diff --git a/drivers/infiniband/hw/mthca/mthca_allocator.c b/drivers/infiniband/hw/mthca/mthca_allocator.c > > > > index aef1d274a14e..1141695093e7 100644 > > > > --- a/drivers/infiniband/hw/mthca/mthca_allocator.c > > > > +++ b/drivers/infiniband/hw/mthca/mthca_allocator.c > > > > @@ -51,6 +51,9 @@ u32 mthca_alloc(struct mthca_alloc *alloc) > > > > } > > > > > > > > if (obj < alloc->max) { > > > > + alloc->last = obj + 1; > > > > + if (alloc->last == alloc->max) > > > > + alloc->last = 0; > > > > set_bit(obj, alloc->table); > > > > obj |= alloc->top; > > > > } else > > > > -- > > > > 2.17.1 > > > > > > [ref] ```