Debug FeatureUpgrading

Trying to debug FeatureUpgrading event receiver on Sharepoint 2013. Attached localy to all w3wp.exe processes. After run

$MyFeature.Upgrade($true)

feature get upgraded, but breakpoint inside FeatureUpgrading wasn't hit. Any ideas?

UPD: upgrading feature with following powershell code:

Update-SPSolution -Identity MySolution.wsp -LiteralPath c:\distr\MySolution.wsp -GACDeployment
$w = Get-SPWeb http://MyPortal/sites/dev
$fn = $w.Features | where{$_.definition.displayname -like '*MySolution*'}
$fn.Upgrade($true)
$fn.Version

At the end I get the new version number from $fn.Version