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s390/wti: Prepare graceful CPU pre-emption on wti reception

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When a warning track interrupt is received, the kernel has only a very
limited amount of time to make sure, that the CPU can be yielded as
gracefully as possible before being pre-empted by the hypervisor.

The interrupt handler for the wti therefore unparks a kernel thread
which has being created on boot re-using the CPU hotplug kernel thread
infrastructure. These threads exist per CPU and are assigned the
highest possible real-time priority. This makes sure, that said threads
will execute as soon as possible as the scheduler should pre-empt any
other running user tasks to run the real-time thread.

Furthermore, the interrupt handler disables all I/O interrupts to
prevent additional interrupt processing on the soon-preempted CPU.
Interrupt handlers are likely to take kernel locks, which in the worst
case, will be kept while the interrupt handler is pre-empted from itself
underlying physical CPU. In that case, all tasks or interrupt handlers
on other CPUs would have to wait for the pre-empted CPU being dispatched
again. By preventing further interrupt processing, this risk is
minimized.

Once the CPU gets dispatched again, the real-time kernel thread regains
control, reenables interrupts and parks itself again.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
This commit is contained in:
Tobias Huschle
2024-08-12 13:39:28 +02:00
committed by Vasily Gorbik
parent 2c6c9ccc76
commit cafeff5a03
2 changed files with 142 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
arch/s390/kernel/Makefile
View File

@@ -43,7 +43,7 @@ obj-y += sysinfo.o lgr.o os_info.o ctlreg.o
obj-y += runtime_instr.o cache.o fpu.o dumpstack.o guarded_storage.o sthyi.o
obj-y += entry.o reipl.o kdebugfs.o alternative.o
obj-y += nospec-branch.o ipl_vmparm.o machine_kexec_reloc.o unwind_bc.o
obj-y += smp.o text_amode31.o stacktrace.o abs_lowcore.o facility.o uv.o
obj-y += smp.o text_amode31.o stacktrace.o abs_lowcore.o facility.o uv.o wti.o
extra-y += vmlinux.lds

141
arch/s390/kernel/wti.c Normal file
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@@ -0,0 +1,141 @@
// SPDX-License-Identifier: GPL-2.0
/*
* Support for warning track interruption
*
* Copyright IBM Corp. 2023
*/
#include <linux/smpboot.h>
#include <linux/irq.h>
#include <uapi/linux/sched/types.h>
#include <asm/diag.h>
#include <asm/sclp.h>
struct wti_state {
/*
* Represents the real-time thread responsible to
* acknowledge the warning-track interrupt and trigger
* preliminary and postliminary precautions.
*/
struct task_struct *thread;
/*
* If pending is true, the real-time thread must be scheduled.
* If not, a wake up of that thread will remain a noop.
*/
bool pending;
};
static DEFINE_PER_CPU(struct wti_state, wti_state);
/*
* During a warning-track grace period, interrupts are disabled
* to prevent delays of the warning-track acknowledgment.
*
* Once the CPU is physically dispatched again, interrupts are
* re-enabled.
*/
static void wti_irq_disable(void)
{
unsigned long flags;
struct ctlreg cr6;
local_irq_save(flags);
local_ctl_store(6, &cr6);
/* disable all I/O interrupts */
cr6.val &= ~0xff000000UL;
local_ctl_load(6, &cr6);
local_irq_restore(flags);
}
static void wti_irq_enable(void)
{
unsigned long flags;
struct ctlreg cr6;
local_irq_save(flags);
local_ctl_store(6, &cr6);
/* enable all I/O interrupts */
cr6.val |= 0xff000000UL;
local_ctl_load(6, &cr6);
local_irq_restore(flags);
}
static void wti_interrupt(struct ext_code ext_code,
unsigned int param32, unsigned long param64)
{
struct wti_state *st = this_cpu_ptr(&wti_state);
inc_irq_stat(IRQEXT_WTI);
wti_irq_disable();
st->pending = true;
wake_up_process(st->thread);
}
static int wti_pending(unsigned int cpu)
{
struct wti_state *st = per_cpu_ptr(&wti_state, cpu);
return st->pending;
}
static void wti_thread_fn(unsigned int cpu)
{
struct wti_state *st = per_cpu_ptr(&wti_state, cpu);
st->pending = false;
/*
* Yield CPU voluntarily to the hypervisor. Control
* resumes when hypervisor decides to dispatch CPU
* to this LPAR again.
*/
diag49c(DIAG49C_SUBC_ACK);
wti_irq_enable();
}
static struct smp_hotplug_thread wti_threads = {
.store = &wti_state.thread,
.thread_should_run = wti_pending,
.thread_fn = wti_thread_fn,
.thread_comm = "cpuwti/%u",
.selfparking = false,
};
static int __init wti_init(void)
{
struct sched_param wti_sched_param = { .sched_priority = MAX_RT_PRIO - 1 };
struct wti_state *st;
int cpu, rc;
rc = -EOPNOTSUPP;
if (!sclp.has_wti)
goto out;
rc = smpboot_register_percpu_thread(&wti_threads);
if (WARN_ON(rc))
goto out;
for_each_online_cpu(cpu) {
st = per_cpu_ptr(&wti_state, cpu);
sched_setscheduler(st->thread, SCHED_FIFO, &wti_sched_param);
}
rc = register_external_irq(EXT_IRQ_WARNING_TRACK, wti_interrupt);
if (rc) {
pr_warn("Couldn't request external interrupt 0x1007\n");
goto out_thread;
}
irq_subclass_register(IRQ_SUBCLASS_WARNING_TRACK);
rc = diag49c(DIAG49C_SUBC_REG);
if (rc) {
pr_warn("Failed to register warning track interrupt through DIAG 49C\n");
rc = -EOPNOTSUPP;
goto out_subclass;
}
goto out;
out_subclass:
irq_subclass_unregister(IRQ_SUBCLASS_WARNING_TRACK);
unregister_external_irq(EXT_IRQ_WARNING_TRACK, wti_interrupt);
out_thread:
smpboot_unregister_percpu_thread(&wti_threads);
out:
return rc;
}
late_initcall(wti_init);