Re: [PATCH] livepatch: Avoid CPU hogging with cond_resched

[Miroslav Benes <mbenes@suse.cz>]

On Wed, 29 Dec 2021, David Vernet wrote:

> When initializing a 'struct klp_object' in klp_init_object_loaded(), and
> performing relocations in klp_resolve_symbols(), klp_find_object_symbol()
> is invoked to look up the address of a symbol in an already-loaded module
> (or vmlinux). This, in turn, calls kallsyms_on_each_symbol() or
> module_kallsyms_on_each_symbol() to find the address of the symbol that is
> being patched.
> 
> It turns out that symbol lookups often take up the most CPU time when
> enabling and disabling a patch, and may hog the CPU and cause other tasks
> on that CPU's runqueue to starve -- even in paths where interrupts are
> enabled.  For example, under certain workloads, enabling a KLP patch with
> many objects or functions may cause ksoftirqd to be starved, and thus for
> interrupts to be backlogged and delayed. This may end up causing TCP
> retransmits on the host where the KLP patch is being applied, and in
> general, may cause any interrupts serviced by softirqd to be delayed while
> the patch is being applied.
> 
> So as to ensure that kallsyms_on_each_symbol() does not end up hogging the
> CPU, this patch adds a call to cond_resched() in kallsyms_on_each_symbol()
> and module_kallsyms_on_each_symbol(), which are invoked when doing a symbol
> lookup in vmlinux and a module respectively.  Without this patch, if a
> live-patch is applied on a 36-core Intel host with heavy TCP traffic, a
> ~10x spike is observed in TCP retransmits while the patch is being applied.
> Additionally, collecting sched events with perf indicates that ksoftirqd is
> awakened ~1.3 seconds before it's eventually scheduled.  With the patch, no
> increase in TCP retransmit events is observed, and ksoftirqd is scheduled
> shortly after it's awakened.
> 
> Signed-off-by: David Vernet <void@manifault.com>

Acked-by: Miroslav Benes <mbenes@suse.cz>

I had similar ideas Petr mentioned elsewhere, but I, also, have no strong 
opinion about it. Especially when livepatch is the only user of the said 
interface.

M