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SubjectRe: [RFC PATCH for 4.21 06/16] cpu_opv: Provide cpu_opv system call (v8)
----- On Oct 17, 2018, at 11:11 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Oct 17, 2018, at 3:19 AM, Srikar Dronamraju srikar@linux.vnet.ibm.com
> wrote:
>
>> Hi Mathieu,
>>
>>> +static int do_cpu_opv(struct cpu_op *cpuop, int cpuopcnt,
>>> + struct cpu_opv_vaddr *vaddr_ptrs, int cpu)
>>> +{
>>> + struct mm_struct *mm = current->mm;
>>> + int ret;
>>> +
>>> +retry:
>>> + if (cpu != raw_smp_processor_id()) {
>>> + ret = push_task_to_cpu(current, cpu);
>>> + if (ret)
>>> + goto check_online;
>>> + }
>>> + down_read(&mm->mmap_sem);
>>> + ret = vaddr_ptrs_check(vaddr_ptrs);
>>> + if (ret)
>>> + goto end;
>>> + preempt_disable();
>>> + if (cpu != smp_processor_id()) {
>>> + preempt_enable();
>>> + up_read(&mm->mmap_sem);
>>> + goto retry;
>>> + }
>>
>> If we have a higher priority task/s either pinned to the cpu, dont we end up
>> in busy-looping till the task exits/sleeps?
>
> You're right!
>
> How about we ditch the thread migration altogether, and simply perform
> the cpu_opv operations in a IPI handler ?
>
> This is possible now that cpu_opv uses a temporary vmap() rather than
> try to touch the user-space page through the current thread's page table.
>
> Thoughts ?

Here is the associated implementation on top of this patchset:

commit 759c5a8860d867091e168900329f0955e5101989
Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed Oct 17 11:32:02 2018 -0400

cpu opv: use ipi

diff --git a/kernel/cpu_opv.c b/kernel/cpu_opv.c
index db144b71d51a..30405e0cc049 100644
--- a/kernel/cpu_opv.c
+++ b/kernel/cpu_opv.c
@@ -31,6 +31,7 @@
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/atomic.h>
+#include <linux/smp.h>
#include <asm/ptrace.h>
#include <asm/byteorder.h>
#include <asm/cacheflush.h>
@@ -1039,41 +1040,48 @@ static int vaddr_ptrs_check(struct cpu_opv_vaddr *vaddr_ptrs)
return 0;
}

+struct opv_ipi_args {
+ struct cpu_op *cpuop;
+ int cpuopcnt;
+ int ret;
+};
+
+static void cpu_opv_ipi(void *info)
+{
+ struct opv_ipi_args *args = info;
+
+ rseq_preempt(current);
+ args->ret = __do_cpu_opv(args->cpuop, args->cpuopcnt);
+}
+
static int do_cpu_opv(struct cpu_op *cpuop, int cpuopcnt,
struct cpu_opv_vaddr *vaddr_ptrs, int cpu)
{
struct mm_struct *mm = current->mm;
+ struct opv_ipi_args args = {
+ .cpuop = cpuop,
+ .cpuopcnt = cpuopcnt,
+ };
int ret;

retry:
- if (cpu != raw_smp_processor_id()) {
- ret = push_task_to_cpu(current, cpu);
- if (ret)
- goto check_online;
- }
+ if (!cpumask_test_cpu(cpu, &current->cpus_allowed))
+ return -EINVAL;
down_read(&mm->mmap_sem);
ret = vaddr_ptrs_check(vaddr_ptrs);
if (ret)
goto end;
- preempt_disable();
- if (cpu != smp_processor_id()) {
- preempt_enable();
+ ret = smp_call_function_single(cpu, cpu_opv_ipi, &args, 1);
+ if (ret) {
up_read(&mm->mmap_sem);
- goto retry;
+ goto check_online;
}
- ret = __do_cpu_opv(cpuop, cpuopcnt);
- preempt_enable();
+ ret = args.ret;
end:
up_read(&mm->mmap_sem);
return ret;

check_online:
- /*
- * push_task_to_cpu() returns -EINVAL if the requested cpu is not part
- * of the current thread's cpus_allowed mask.
- */
- if (ret == -EINVAL)
- return ret;
get_online_cpus();
if (cpu_online(cpu)) {
put_online_cpus();



--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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