memory allocation in spinlock context
I am trying to understand if it is possible to allow memory allocations (M_NOWAIT, of course) in a spinlock context. I do not see any obvious architectural obstacles. But the fact that all of the uma locks, system map lock, object locks, page queue locks and so on are regular mutexes makes it impossible to allocate memory without violating the fundamental lock ordering rules. Could all of the above mentioned locks potentially be converted to spin mutexes? (And that seems to be a large nasty change) Are there any alternative possibilities? BTW, currently we have at least one place where a memory allocation of this kind is done stealthily (and thus dangerously?). ACPI resume code must execute AcpiLeaveSleepStatePrep with interrupts disabled and ACPICA code performs memory allocations in that code path. Since the interrupts are disabled by means of intr_disable(), witness(9) and similar are completely oblivious of the fact. -- Andriy Gapon ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: memory allocation in spinlock context
On 3/1/2013 5:50 AM, Andriy Gapon wrote: I am trying to understand if it is possible to allow memory allocations (M_NOWAIT, of course) in a spinlock context. There are mechanisms to do just this- essentially by creating private pools that are organized in a way to allow for spinlock (and thus possible interrupt level) safe allocations (and failure if the pool is empty). Are you trying to make a general mechanism for this? ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: memory allocation in spinlock context
On Fri, Mar 1, 2013 at 2:50 PM, Andriy Gapon a...@freebsd.org wrote: I am trying to understand if it is possible to allow memory allocations (M_NOWAIT, of course) in a spinlock context. I do not see any obvious architectural obstacles. But the fact that all of the uma locks, system map lock, object locks, page queue locks and so on are regular mutexes makes it impossible to allocate memory without violating the fundamental lock ordering rules. Could all of the above mentioned locks potentially be converted to spin mutexes? (And that seems to be a large nasty change) AFAIK they're suitable for particular uses and not in general. For example if the critical section is short, spinning rather than sleeping could avoid a potential context switches, increasing performances. OTOH has the disadvantage of wasting time that could be used in other activities. So, IMHO, if such a change need to be adopted, ti should be pondered/profiled more than a bit, and I doubt it could be used for the wide class of locks you mentioned. Are there any alternative possibilities? Is there anything that prevent you to drop the lock, perform the allocation in a reliable fashion (M_WAITOK) and try to reacquire the lock later on? BTW, currently we have at least one place where a memory allocation of this kind is done stealthily (and thus dangerously?). ACPI resume code must execute AcpiLeaveSleepStatePrep with interrupts disabled and ACPICA code performs memory allocations in that code path. Since the interrupts are disabled by means of intr_disable(), witness(9) and similar are completely oblivious of the fact. -- Andriy Gapon ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org Thanks, -- Davide ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: memory allocation in spinlock context
on 01/03/2013 16:22 Matthew Jacob said the following: On 3/1/2013 5:50 AM, Andriy Gapon wrote: I am trying to understand if it is possible to allow memory allocations (M_NOWAIT, of course) in a spinlock context. There are mechanisms to do just this- essentially by creating private pools that are organized in a way to allow for spinlock (and thus possible interrupt level) safe allocations (and failure if the pool is empty). Are you trying to make a general mechanism for this? I am just pondering what would it take to develop such a general mechanism. -- Andriy Gapon ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
Re: memory allocation in spinlock context
On 3/1/13 5:50 AM, Andriy Gapon wrote: I am trying to understand if it is possible to allow memory allocations (M_NOWAIT, of course) in a spinlock context. I do not see any obvious architectural obstacles. But the fact that all of the uma locks, system map lock, object locks, page queue locks and so on are regular mutexes makes it impossible to allocate memory without violating the fundamental lock ordering rules. Could all of the above mentioned locks potentially be converted to spin mutexes? (And that seems to be a large nasty change) Are there any alternative possibilities? BTW, currently we have at least one place where a memory allocation of this kind is done stealthily (and thus dangerously?). ACPI resume code must execute AcpiLeaveSleepStatePrep with interrupts disabled and ACPICA code performs memory allocations in that code path. Since the interrupts are disabled by means of intr_disable(), witness(9) and similar are completely oblivious of the fact. Typically the need for such a facility means that the locks are being held for too long. I think someone has suggested using a private allocator carving out of a pre-allocated space. Depending on the subsystem you are allocating for this may work for you. I am looking to do this for the kernel gzip routines so that we can do compressed kernel dumps as soon as I verify the bounds of the gzip allocations. -Alfred ___ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to freebsd-hackers-unsubscr...@freebsd.org
