From: Xu Kuohai <[email protected]>
pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
free nodes for some CPUs, and then possibly one CPU with fewer nodes,
followed by remaining cpus with 0 nodes. For example, when nr_elems == 256
and num_possible_cpus() == 32, if CPU 0 is the current cpu, CPU 0~27
each gets 9 free nodes, CPU 28 gets 4 free nodes, CPU 29~31 get 0 free
nodes, while in fact each CPU should get 8 nodes equally.
This patch initializes nr_elems / num_possible_cpus() free nodes for each
CPU firstly, then allocates the remaining free nodes by one for each CPU
until no free nodes left.
Signed-off-by: Xu Kuohai <[email protected]>
Acked-by: Yonghong Song <[email protected]>
---
v2: Update commit message and add Yonghong's ack
---
kernel/bpf/percpu_freelist.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
index b6e7f5c5b9ab..89e84f7381cc 100644
--- a/kernel/bpf/percpu_freelist.c
+++ b/kernel/bpf/percpu_freelist.c
@@ -100,12 +100,15 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
u32 nr_elems)
{
struct pcpu_freelist_head *head;
- int i, cpu, pcpu_entries;
+ int i, cpu, pcpu_entries, remain_entries;
+
+ pcpu_entries = nr_elems / num_possible_cpus();
+ remain_entries = nr_elems % num_possible_cpus();
- pcpu_entries = nr_elems / num_possible_cpus() + 1;
i = 0;
for_each_possible_cpu(cpu) {
+ int j = i + pcpu_entries + (remain_entries-- > 0 ? 1 : 0);
again:
head = per_cpu_ptr(s->freelist, cpu);
/* No locking required as this is not visible yet. */
@@ -114,7 +117,7 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
buf += elem_size;
if (i == nr_elems)
break;
- if (i % pcpu_entries)
+ if (i < j)
goto again;
}
}
--
2.30.2
On Tue, Nov 8, 2022 at 6:05 AM Xu Kuohai <[email protected]> wrote:
>
> From: Xu Kuohai <[email protected]>
>
> pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
> free nodes for some CPUs, and then possibly one CPU with fewer nodes,
> followed by remaining cpus with 0 nodes. For example, when nr_elems == 256
> and num_possible_cpus() == 32, if CPU 0 is the current cpu, CPU 0~27
> each gets 9 free nodes, CPU 28 gets 4 free nodes, CPU 29~31 get 0 free
> nodes, while in fact each CPU should get 8 nodes equally.
>
> This patch initializes nr_elems / num_possible_cpus() free nodes for each
> CPU firstly, then allocates the remaining free nodes by one for each CPU
> until no free nodes left.
>
> Signed-off-by: Xu Kuohai <[email protected]>
> Acked-by: Yonghong Song <[email protected]>
> ---
> v2: Update commit message and add Yonghong's ack
> ---
> kernel/bpf/percpu_freelist.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
> index b6e7f5c5b9ab..89e84f7381cc 100644
> --- a/kernel/bpf/percpu_freelist.c
> +++ b/kernel/bpf/percpu_freelist.c
> @@ -100,12 +100,15 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
> u32 nr_elems)
> {
> struct pcpu_freelist_head *head;
> - int i, cpu, pcpu_entries;
> + int i, cpu, pcpu_entries, remain_entries;
> +
> + pcpu_entries = nr_elems / num_possible_cpus();
> + remain_entries = nr_elems % num_possible_cpus();
>
> - pcpu_entries = nr_elems / num_possible_cpus() + 1;
> i = 0;
>
> for_each_possible_cpu(cpu) {
> + int j = i + pcpu_entries + (remain_entries-- > 0 ? 1 : 0);
> again:
> head = per_cpu_ptr(s->freelist, cpu);
> /* No locking required as this is not visible yet. */
> @@ -114,7 +117,7 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
> buf += elem_size;
> if (i == nr_elems)
> break;
> - if (i % pcpu_entries)
> + if (i < j)
> goto again;
> }
this loop's logic is quite hard to follow, if we are fixing it, can we
simplify it maybe? something like:
int cpu, cpu_idx, i, j, n, m;
n = nr_elems / num_possible_cpus();
m = nr_elems % num_possible_cpus();
for_each_possible_cpu(cpu) {
i = n + (cpu_idx < m ? 1 : 0);
for (j = 0; j < i; j++) {
head = per_cpu_ptr(s->freelist, cpu);
pcpu_freelist_push_node(head, buf);
buf += elem_size;
}
cpu_idx++;
}
no gotos, no extra ifs: for each cpu we determine correct number of
elements to allocate, then just allocate them in a straightforward
loop
> }
> --
> 2.30.2
>
On 11/10/2022 7:56 AM, Andrii Nakryiko wrote:
> On Tue, Nov 8, 2022 at 6:05 AM Xu Kuohai <[email protected]> wrote:
>>
>> From: Xu Kuohai <[email protected]>
>>
>> pcpu_freelist_populate() initializes nr_elems / num_possible_cpus() + 1
>> free nodes for some CPUs, and then possibly one CPU with fewer nodes,
>> followed by remaining cpus with 0 nodes. For example, when nr_elems == 256
>> and num_possible_cpus() == 32, if CPU 0 is the current cpu, CPU 0~27
>> each gets 9 free nodes, CPU 28 gets 4 free nodes, CPU 29~31 get 0 free
>> nodes, while in fact each CPU should get 8 nodes equally.
>>
>> This patch initializes nr_elems / num_possible_cpus() free nodes for each
>> CPU firstly, then allocates the remaining free nodes by one for each CPU
>> until no free nodes left.
>>
>> Signed-off-by: Xu Kuohai <[email protected]>
>> Acked-by: Yonghong Song <[email protected]>
>> ---
>> v2: Update commit message and add Yonghong's ack
>> ---
>> kernel/bpf/percpu_freelist.c | 9 ++++++---
>> 1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/bpf/percpu_freelist.c b/kernel/bpf/percpu_freelist.c
>> index b6e7f5c5b9ab..89e84f7381cc 100644
>> --- a/kernel/bpf/percpu_freelist.c
>> +++ b/kernel/bpf/percpu_freelist.c
>> @@ -100,12 +100,15 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
>> u32 nr_elems)
>> {
>> struct pcpu_freelist_head *head;
>> - int i, cpu, pcpu_entries;
>> + int i, cpu, pcpu_entries, remain_entries;
>> +
>> + pcpu_entries = nr_elems / num_possible_cpus();
>> + remain_entries = nr_elems % num_possible_cpus();
>>
>> - pcpu_entries = nr_elems / num_possible_cpus() + 1;
>> i = 0;
>>
>> for_each_possible_cpu(cpu) {
>> + int j = i + pcpu_entries + (remain_entries-- > 0 ? 1 : 0);
>> again:
>> head = per_cpu_ptr(s->freelist, cpu);
>> /* No locking required as this is not visible yet. */
>> @@ -114,7 +117,7 @@ void pcpu_freelist_populate(struct pcpu_freelist *s, void *buf, u32 elem_size,
>> buf += elem_size;
>> if (i == nr_elems)
>> break;
>> - if (i % pcpu_entries)
>> + if (i < j)
>> goto again;
>> }
>
> this loop's logic is quite hard to follow, if we are fixing it, can we
> simplify it maybe? something like:
>
> int cpu, cpu_idx, i, j, n, m;
>
> n = nr_elems / num_possible_cpus();
> m = nr_elems % num_possible_cpus();
>
> for_each_possible_cpu(cpu) {
> i = n + (cpu_idx < m ? 1 : 0);
> for (j = 0; j < i; j++) {
> head = per_cpu_ptr(s->freelist, cpu);
> pcpu_freelist_push_node(head, buf);
> buf += elem_size;
> }
> cpu_idx++;
> }
>
>
> no gotos, no extra ifs: for each cpu we determine correct number of
> elements to allocate, then just allocate them in a straightforward
> loop
>
that's great, will update to:
int cpu, cpu_idx, i, j, n, m;
n = nr_elems / num_possible_cpus();
m = nr_elems % num_possible_cpus();
for_each_possible_cpu(cpu) {
j = min(n + (cpu_idx < m ? 1 : 0), nr_elems);
for (i = 0; i < j; i++) {
head = per_cpu_ptr(s->freelist, cpu);
pcpu_freelist_push_node(head, buf);
buf += elem_size;
}
nr_elems -= j;
cpu_idx++;
}
>> }
>> --
>> 2.30.2
>>