git: 248f0cabca75 - main - make maximum interrupt number tunable on ARM, ARM64, MIPS, and RISC-V

[Michal Meloun]

On 19.01.2021 19:41, John Baldwin wrote:
> On 1/19/21 8:43 AM, Jessica Clarke wrote:
>> On 19 Jan 2021, at 00:36, Oleksandr Tymoshenko <gonzo at FreeBSD.org> wrote:
>>> @@ -142,21 +143,15 @@ static bool irq_assign_cpu = false;
>>> #endif
>>> #endif
>>>
>>> -/*
>>> - * - 2 counters for each I/O interrupt.
>>> - * - MAXCPU counters for each IPI counters for SMP.
>>> - */
>>> -#ifdef SMP
>>> -#define INTRCNT_COUNT   (NIRQ * 2 + INTR_IPI_COUNT * MAXCPU)
>>> -#else
>>> -#define INTRCNT_COUNT   (NIRQ * 2)
>>> -#endif
>>> +int intr_nirq = NIRQ;
>>> +SYSCTL_UINT(_machdep, OID_AUTO, nirq, CTLFLAG_RDTUN, &intr_nirq, 0,
>>> +    "Number of IRQs");
>>
>> Unsigned integer, given the SYSCTL_UINT?
>>
>> What's stopping us from dynamically resizing rather than forcing the
>> user to use a tunable (which also means that, in practice, the limit
>> will remain unchanged, because you want GENERIC kernels to work out of
>> the box)?
> 
> For x86 (and probably the same here), the biggest headache is that there
> is a dynamically allocated array indexed by IRQ value at interrupt
> invocation time.  Currently it uses no locks since the value of the
> pointer is set during boot time and then never changes.  Making it
> dynamic would require allowing that pointer to change and dealing with
> what that entails in a very hot path.  Generally speaking, the majority
> of interrupts used in a given system are allocated during boot and not
> by hotplug devices added post-boot, so there is also not a lot of utility
> in having this run-time tunable vs a boot-time tunable.
> 
To be exact, the main headaches are  intrcnt and intrnames arrays and 
their relationship to the sysctl interface. These fields are used 
without locking, they do not have the ability to mark interrupt at given 
index as unused. This is the real reason why we can't unload a driver 
that acts as an interrupt controller (e.g. USB parallel port with the 
possibility of interrupts on each pin).
Everything else in subr_intr.c should be prepared for dynamic 
allocation/delete of interrupt sources.
Michal